RSE: Use common cpuarch.cmake for NS

All RSE platforms just redirect the build system to a common
cpuarch.cmake file without overriding any of the variables. Therefore,
there is no reason for the NS build to include the platform specific
file which will just loop back to the common file; this also leads to
path issues.

For the NS build, just include the common cpuarch.cmake file directory.
This allows us to remove the install step for the platform specific
cpuarch.cmake/config.cmake files as they are no longer used.

Change-Id: Ibf9e2918134707d7e443c8b311c056124d18245e
Signed-off-by: Jackson Cooper-Driver <jackson.cooper-driver@arm.com>