feat(spmc): enable checking of execution ctx count

This is a mandatory entry in an SP's manifest however
currently an S-EL1 partition running under the EL3 SPMC
must have the same amount of execution contexts as
physical cores therefore just check the entry matches
this value.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I4c2a85ccde7a7bb9b1232cf6389a8c532cbf3d41