infra/rdn2cfg2: add manifest for rdn2cfg2 platform

The RD-N2-Cfg2 is a quad-chip version of the RD-N2 platform. The
Neoverse Reference Design platform software stack now includes support
for this platform. Add the manifest file for this new platform.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I8f96e29321aae520a1f0ca4308ce924ec4f9ed54