sh: machine_ops based reboot support.

This provides a machine_ops-based reboot interface loosely cloned from
x86, and converts the native sh32 and sh64 cases over to it.

Necessary both for tying in SMP support and also enabling platforms like
SDK7786 to add support for their microcontroller-based power managers.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>