[POWERPC] 83xx: Cleaning up machine probing and board initcalls

Cleaned up the probing functionality to be more consistent across all 83xx
boards and added machine_is() protection around board initcalls to ensure they
only do something if we are actually running on that board.

Additionally, removed some dead code on mpc832x_mds.

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