SPMC: adjust the number of EC context to max number of PEs

SPMC: adjust the number of EC context to max number of PEs

According to [1] and in context of FF-A v1.0 a secure partition must
have either one EC (migratable UP) or a number of ECs equal to the
number of PEs (pinned MP). Adjust the SPMC manifest such that the
number of ECs is equal to the number of PEs.

[1] https://trustedfirmware-a.readthedocs.io/en/latest/components/
secure-partition-manager.html#platform-topology

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ie8c7d96ae7107cb27f5b97882d8f476c18e026d4