[POWERPC] PS3: Frame buffer system-bus rework

Convert the ps3fb device from a platform device to a PS3 system bus device.
Fix the remove and shutdown methods to support kexec and to make ps3fb a
loadable module.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>