feat(board/rdedmunds): add support for rdedmunds variant

Add initial support for RD-Edmunds platform. This platform is considered
as a variant of RD-N2 platform with only major change being the CPU
which is Demeter instead of Neoverse-N2.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I939d9eac652fa9e76ad002ee5e6107aa79baa013