feat(spmd): update SPMC init flow to use EL3 implementation

feat(spmd): update SPMC init flow to use EL3 implementation

Allow the SPMD to initialise an SPMC implementation at EL3 directly
rather than at a lower EL.
This includes removing the requirement to parse an SPMC manifest to
obtain information about the SPMC implementation, in this case since the
SPMD and SPMC reside in the same EL we can hardcode the required
information directly.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I66d1e1b3ec2d0abbfc28b011a32445ee890a331d