[POWERPC] XilinxFB: Split device setup from bus binding

Split the device setup code away from the platform bus binding.  This is
in preparation for adding the of_platform bus binding to this driver and
most of the setup code is common between the two busses.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Andrei Konovalov <akonovalov@ru.mvista.com>