Convert shutdownhook_establish() to pmf_device_register1().

XXX  This should be done as part of an overall plan to support
power management and device detachment.  However, in order to do
that, I would first have to invent sbus_intr_disestablish().  This
is being done at this time in order to aid in the effort to eliminate
shutdownhook_establish().

(jnemeth)