infra: add a manifest for RD-N2-Cfg2 platform

Add a new manifest for the RD-N2-Cfg2 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: Ibb94888e524499cd932a382f0c393d995f045be4