mpc85xx: support board-specific reset function

This is useful for boards which cannot be reset in the usual way for the
85xx CPU. An example is a board which can only be reset by a hardware
watchdog.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>