powerpc/powernv: Add OPAL soft-poweroff routine

Register a notifier for a OPAL message indicating that the machine
should prepare itself for a graceful power off.

OPAL will tell us if the power off is a reboot or shutdown, but for now
we perform the same orderly_poweroff action.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>