arm_ffa: introduce armffa command

Provide armffa command showcasing the use of the U-Boot FF-A support

armffa is a command showcasing how to invoke FF-A operations.
This provides a guidance to the client developers on how to
call the FF-A bus interfaces. The command also allows to gather secure
partitions information and ping these  partitions. The command is also
helpful in testing the communication with secure partitions.

For more details please refer to the command documentation [1].

[1]: doc/usage/cmd/armffa.rst

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>