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/MAINTAINERS b/MAINTAINERS
index 9d59ac0..5a2a546 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -274,6 +274,7 @@
 F:	doc/arch/arm64.ffa.rst
 F:	drivers/firmware/arm-ffa/
 F:	include/arm_ffa.h
+F:	test/dm/ffa.c
 
 ARM FREESCALE IMX
 M:	Stefano Babic <sbabic@denx.de>