[POWERPC] Create and hook up of_platform_device_shutdown

Although of_platform_device's can have a shutdown routine, at the moment
the bus code doesn't actually call it. So add the required glue to
hook the shutdown routine.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>