cmake: product/rdn2

This change adds CMake build support for rdn2 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.

Change-Id: I65fc3d014fe0482b623da96833d69b7e7f42f96f
Signed-off-by: Girish Pathak <girish.pathak@arm.com>