arm_ffa: introduce sandbox FF-A support

Emulate Secure World's FF-A ABIs and allow testing U-Boot FF-A support

Features of the sandbox FF-A support:

- Introduce an FF-A emulator
- Introduce an FF-A device driver for FF-A comms with emulated Secure World
- Provides test methods allowing to read the status of the inspected ABIs

The sandbox FF-A emulator supports only 64-bit direct messaging.

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 fcca162..9d59ac0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -269,10 +269,11 @@
 ARM FF-A
 M:	Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
 S:	Maintained
+F:	arch/sandbox/include/asm/sandbox_arm_ffa.h
+F:	arch/sandbox/include/asm/sandbox_arm_ffa_priv.h
 F:	doc/arch/arm64.ffa.rst
 F:	drivers/firmware/arm-ffa/
 F:	include/arm_ffa.h
-F:	include/sandbox_arm_ffa.h
 
 ARM FREESCALE IMX
 M:	Stefano Babic <sbabic@denx.de>