Handle calls to the FFA_SPM_ID_GET interface. If FFA_SPM_ID_GET is
invoked from the non-secure physical FF-A instance, return the SPMC id
(defined in the SPMC manifest). If FFA_SPM_ID_GET is invoked from
the secure physical FF-A instance (e.g. the SPMC), return the SPMD id.

Change-Id: Id6d4e96b1da2510386d344e09c4553dba01227ec
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>