Documentation: dt: Add bindings for Secure-only devices

The existing device tree bindings assume that we are only trying to
describe a single address space with a device tree (for ARM, either
the Normal or the Secure world). Some uses for device tree need to
describe both Normal and Secure worlds in a single device tree. Add
documentation of how to do this, by adding extra properties which
describe when a device appears differently in the two worlds or when
it only appears in one of them.

The binding describes the general principles for adding new
properties describing the secure world, but for now we only need a
single new property, "secure-status", which can be used to annotate
devices to indicate that they are only visible in one of the two
worlds.

The primary expected use of this binding is for a virtual machine
like QEMU to describe the VM layout to a TrustZone aware firmware
(which would then use the secure-only devices itself, and pass the DT
on to a kernel running in the non-secure world, which ignores the
secure-only devices and uses the rest).

Signed-off-by: Peter Maydell <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/arm/secure.txt

···
+* ARM Secure world bindings
+
+ARM CPUs with TrustZone support have two distinct address spaces,
+"Normal" and "Secure". Most devicetree consumers (including the Linux
+kernel) are not TrustZone aware and run entirely in either the Normal
+world or the Secure world. However some devicetree consumers are
+TrustZone aware and need to be able to determine whether devices are
+visible only in the Secure address space, only in the Normal address
+space, or visible in both. (One example of that situation would be a
+virtual machine which boots Secure firmware and wants to tell the
+firmware about the layout of the machine via devicetree.)
+
+The general principle of the naming scheme for Secure world bindings
+is that any property that needs a different value in the Secure world
+can be supported by prefixing the property name with "secure-". So for
+instance "secure-foo" would override "foo". For property names with
+a vendor prefix, the Secure variant of "vendor,foo" would be
+"vendor,secure-foo". If there is no "secure-" property then the Secure
+world value is the same as specified for the Normal world by the
+non-prefixed property. However, only the properties listed below may
+validly have "secure-" versions; this list will be enlarged on a
+case-by-case basis.
+
+Defining the bindings in this way means that a device tree which has
+been annotated to indicate the presence of Secure-only devices can
+still be processed unmodified by existing Non-secure software (and in
+particular by the kernel).
+
+Note that it is still valid for bindings intended for purely Secure
+world consumers (like kernels that run entirely in Secure) to simply
+describe the view of Secure world using the standard bindings. These
+secure- bindings only need to be used where both the Secure and Normal
+world views need to be described in a single device tree.
+
+Valid Secure world properties:
+
+- secure-status : specifies whether the device is present and usable
+  in the secure world. The combination of this with "status" allows
+  the various possible combinations of device visibility to be
+  specified. If "secure-status" is not specified it defaults to the
+  same value as "status"; if "status" is not specified either then
+  both default to "okay". This means the following combinations are
+  possible:
+
+   /* Neither specified: default to visible in both S and NS */
+   secure-status = "okay";                          /* visible in both */
+   status = "okay";                                 /* visible in both */
+   status = "okay"; secure-status = "okay";         /* visible in both */
+   secure-status = "disabled";                      /* NS-only */
+   status = "okay"; secure-status = "disabled";     /* NS-only */
+   status = "disabled"; secure-status = "okay";     /* S-only */
+   status = "disabled";                             /* disabled in both */
+   status = "disabled"; secure-status = "disabled"; /* disabled in both */