scmi: separate scmi base protocol

Move all SCMI Base Protocol related functions to separate files.

Add the posibility to include (or not) the SCMI Base Protocol related
functionality. This will improve memory usage in those cases where the
SCMI Base Protocol is unnecessary, and will avoid adding the overhead
caused by creating a new module for the SCMI Base Protocol.

The CMake variable SCP_TARGET_EXCLUDE_BASE_PROTOCOL is introduced to
avoid including the SCMI Base Protocol functionality in the code.
In order to avoid the inclusion, just use the command
set(SCP_TARGET_EXCLUDE_BASE_PROTOCOL TRUE) in the Firmware.cmake file
of the specific target.

Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>
Change-Id: I90a81c55a6c22e8f4f4b699b9e4c4139065db4bc