Dualcpu: Move dual-cpu NS files into a dedicated folder

Gather all the dual-cpu NS .c files in a dedicated folder
multi_core under interface/src.
Put all the dual-cpu interface header files in a dedicated folder
multi-core under interface/include.

Change-Id: Ia6847eb03a663e66260690ad88addb1521f13fcd
Signed-off-by: David Hu <david.hu@arm.com>
10 files changed