module/scmi: create new API for initiating SCMI messages

Add new function that sends an SCMI message given the message
parameters, scmi service id, etc. Also expose this function as a part
of mod_scmi_from_protocol_api. Any module that wants to initiate SCMI
communication can bind to this API and provide appropriate SCMI message
handler function pointers back to scmi module.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: I9c51894891c74fca34b5cd9049c5a55063f995e5