[POWERPC] Search for and publish cell OF platform devices earlier

Currently cell publishes OF devices at device_initcall() time, which
means the earliest a driver can bind to a device is also device_initcall()
time.  We have a driver we want to register before other devices, so
publish the devices at subsys_initcall() time.

This should not cause any behaviour change for existing drivers, as they
are still bound at device_initcall() time.

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>