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/configs/sandbox_defconfig b/configs/sandbox_defconfig
index ea2b256..1cd1c2e 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -345,3 +345,4 @@
 CONFIG_UNIT_TEST=y
 CONFIG_UT_TIME=y
 CONFIG_UT_DM=y
+CONFIG_ARM_FFA_TRANSPORT=y