feat(spmc): add support for FFA_SPM_ID_GET

Enable a Secure Partition to query the ID assigned to the SPMC.
The SPMD will take care of any calls from the normal world
therefore we should not need to handle this case in the SPMC.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I97903e920e928df385addbb2d383f24e602bf2db