feat(spmc): add FF-A secure partition manager core

feat(spmc): add FF-A secure partition manager core

This patch introduces the core support for enabling an SPMC in EL3
as per the FF-A spec.

The current implemented functionality is targeted to enable
initialization of the SPMC itself and initial support for
bringing up a single S-EL1 SP.

This includes initialization of the SPMC's internal state,
parsing of an SP's manifest, preparing the cpu contexts and
appropriate system registers for the Secure Partition.

The spmc_smc_handler is the main handler for all incoming SMCs
to the SPMC, FF-A ABI handlers and functionality will
be implemented in subsequent patches.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: Ib33c240b91e54cbd018a69fec880d02adfbe12b9