cmake: product/n1sdp

This change adds CMake build support for n1sdp platform

Please note that firmware targets do not use libRTX_CM3.a
or RTX_CM3.lib, instead in CMake build, the library
is built using sources.

Signed-off-by: Girish Pathak <girish.pathak@arm.com>
Change-Id: I5daf93343bb7a2c320c29b7252d000dc08c7e6c4