drivers/scmi-msg: smt entry points for incoming messages

drivers/scmi-msg: smt entry points for incoming messages

This change implements SCMI channels for reading a SCMI message from a
shared memory and call the SCMI message drivers to route the message
to the target platform services.

SMT refers to the shared memory management protocol which is used
to get/put message/response in shared memory. SMT is a 28byte header
stating shared memory state and exchanged protocol data.

The processing entry for a SCMI message can be a secure interrupt
or fastcall SMCCC invocation.

SMT description in this implementation is based on the OP-TEE
project [1] itself based in the SCP-firmware implementation [2].

Link: [1] https://github.com/OP-TEE/optee_os/commit/a58c4d706d2333d2b21a3eba7e2ec0cb257bca1d
Link: [2] https://github.com/ARM-software/SCP-firmware.git

Change-Id: I416c7dab5c67954c6fe80bae8d8cdfdcda66873e
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>