Twincpu: Abstract configure_ns_code() for diverse topologies

Declare configure_ns_core() in tfm_nspm.h.
Move single Armv8-M specific configure_ns_code() implementation to
tfm_nspm_ipc.c/tfm_nspm_func.c.
Define an empty configure_ns_code() for multi-core topology.

Change-Id: If35570b4d23f9795c3efd16ab8a2b18a30c4e821
Signed-off-by: David Hu <david.hu@arm.com>