feat(spmc): enable handling of the NS bit

In FF-A v1.1 the NS bit is used by the SPMC to specify the
security state of a memory region retrieved by a SP.

Enable the SPMC to set the bit for v1.1 callers or v1.0
callers that explicitly request the usage via FFA_FEATURES.

In this implementation the sender of the memory region must
reside in the normal world and the SPMC does not support
changing the security state of memory regions therefore
always set the NS bit if required by the caller.

Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Change-Id: I215756b28e2382082933ba1dcc7584e7faf4b36b