docs: Xen ARM DT bindings

Add a doc to describe the Xen ARM device tree bindings


Changes in v5:

- add a comment about the size of the grant table memory region;
- add a comment about the required presence of a GIC node;
- specify that the described properties are part of a top-level
"hypervisor" node;
- specify #address-cells and #size-cells for the example.


Changes in v4:

- "xen,xen" should be last as it is less specific;
- update reg property using 2 address-cells and 2 size-cells.


Signed-off-by: Stefano Stabellini <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
CC: [e-mail]
CC: David Vrabel <[e-mail]>
CC: Rob Herring <[e-mail]>
CC: Dave Martin <[e-mail]>

Documentation/devicetree/bindings/arm/xen.txt

···
+* Xen hypervisor device tree bindings
+
+Xen ARM virtual platforms shall have a top-level "hypervisor" node with
+the following properties:
+
+- compatible:
+	compatible = "xen,xen-<version>", "xen,xen";
+  where <version> is the version of the Xen ABI of the platform.
+
+- reg: specifies the base physical address and size of a region in
+  memory where the grant table should be mapped to, using an
+  HYPERVISOR_memory_op hypercall. The memory region is large enough to map
+  the whole grant table (it is larger or equal to gnttab_max_grant_frames()).
+
+- interrupts: the interrupt used by Xen to inject event notifications.
+  A GIC node is also required.
+
+
+Example (assuming #address-cells = <2> and #size-cells = <2>):
+
+hypervisor {
+	compatible = "xen,xen-4.3", "xen,xen";
+	reg = <0 0xb0000000 0 0x20000>;
+	interrupts = <1 15 0xf08>;
+};