[POWERPC] 85xx: Renamed MPC8568 MDS board code to match other boards

Renamed the MPC8568 MDS platform code to follow other 85xx boards.  There
isn't anything specific about the 8568 MDS code that wouldn't apply to another
85xx MDS system at this point.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>