StandaloneMmPkg: Allow sending FFA Direct Request message to StandaloneMm

StandaloneMmPkg: Allow sending FFA Direct Request message to StandaloneMm

Allow passing of a request to StandaloneMm Core through the Firmware
Framework(FF-A) using FFA_MSG_SEND_DIRECT_REQ method. This method is
used as a mechanism for requesting some service from StandaloneMm.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>