docs: add Arm FF-A mediator

Describes a FF-A version 1.1 [1] mediator to communicate with a Secure
Partition in secure world.

[1] https://developer.arm.com/documentation/den0077/latest

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

SUPPORT.md

@@ -842,6 +842,15 @@ that covers the DMA of the device to be passed through.
 
 No support for QEMU backends in a 16K or 64K domain.
 
+### ARM: Firmware Framework for Arm A-profile (FF-A) Mediator
+
+    Status, Arm64: Tech Preview
+
+There are still some code paths where a vCPU may hog a pCPU longer than
+necessary. The FF-A mediator is not yet implemented for Arm32. Part of the
+FF-A specification is not supported, see the top comment in
+xen/arch/arm/tee/ffa.c for limitations.
+
 ### ARM: Guest Device Tree support
 
     Status: Supported