arm_ffa: introduce sandbox test cases for UCLASS_FFA

Add functional test cases for the FF-A support

These tests rely on the FF-A sandbox emulator and FF-A
sandbox driver which help in inspecting the FF-A communication.

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/arch/arm64.ffa.rst b/doc/arch/arm64.ffa.rst
index 7928983..7160637 100644
--- a/doc/arch/arm64.ffa.rst
+++ b/doc/arch/arm64.ffa.rst
@@ -37,6 +37,7 @@
   FF-A ABIs inspection methods.
 - An FF-A sandbox device driver for FF-A communication with the emulated Secure World.
   The driver leverages the FF-A Uclass to establish FF-A communication.
+- Sandbox FF-A test cases.
 
 FF-A and SMC specifications
 -------------------------------------------