This change introduces drivers to allow a platform to create a basic
SCMI service and register handlers for client request (SCMI agent) on
system resources. This is the first piece of the drivers: an entry
function, the SCMI base protocol support and helpers for create
the response message.

With this change, scmi_process_message() is the entry function to
process an incoming SCMI message. The function expect the message
is already copied from shared memory into secure memory. The message
structure stores message reference and output buffer reference where
response message shall be stored.

scmi_process_message() calls the SCMI protocol driver according to
the protocol ID in the message. The SCMI protocol driver will call
defined platform handlers according to the message content.

This change introduces only the SCMI base protocol as defined in
SCMI specification v2.0 [1]. Not all the messages defined
in the specification are supported.

The SCMI message implementation is derived from the OP-TEE project [2]
itself based on the SCP-firmware implementation [3] of the SCMI protocol
server side.

Link: [1] http://infocenter.arm.com/help/topic/com.arm.doc.den0056a/DEN0056A_System_Control_and_Management_Interface.pdf
Link: [2] https://github.com/OP-TEE/optee_os/commit/ae8c8068098d291e6e55744dbc237ec39fd9840a
Link: [3] https://github.com/ARM-software/SCP-firmware/tree/v2.6.0

Change-Id: I639c4154a39fca60606264baf8d32452641f45e9
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>