infra: add a manifest for RD-N2 platform

Add a new manifest for the RD-N2 platform that tracks the development
branch. This is different from the pinned version of the manifest and
allows updates on the development branch to be used.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I9039de49aac7f6431416d752cf54d3552a3f3767