of: add Trusted Foundations bindings documentation

Add the Device Tree bindings documentation for the Trusted Foundation
secure monitor.

Signed-off-by: Alexandre Courbot <[e-mail]>
Signed-off-by: Stephen Warren <[e-mail]>

Documentation/devicetree/bindings/arm/firmware/tlm,trusted-foundations.txt

···
+Trusted Foundations
+-------------------
+
+Boards that use the Trusted Foundations secure monitor can signal its
+presence by declaring a node compatible with "tlm,trusted-foundations"
+under the /firmware/ node
+
+Required properties:
+- compatible: "tlm,trusted-foundations"
+- tlm,version-major: major version number of Trusted Foundations firmware
+- tlm,version-minor: minor version number of Trusted Foundations firmware
+
+Example:
+	firmware {
+		trusted-foundations {
+			compatible = "tlm,trusted-foundations";
+			tlm,version-major = <2>;
+			tlm,version-minor = <8>;
+		};
+	};