powerpc/85xx: clean up for mpc8568_mds name

Keep an unique machine def for the MPC8568 MDS board to handle some
subtle differences between the future MDS board. Also set the bcsrs in
setup_arch() only for mpc8568_mds because other mds has different bcsr
settings.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>