[POWERPC] spufs: split the Cell BE support into generic and platform dependant parts

Creates new config variables PPC_CELL_NATIVE and PPC_IBM_CELL_BLADE.
The existing CONFIG_PPC_CELL is now used to denote the generic
Cell processor support.

PPC_CELL = make descends into platforms/cell
PPC_CELL_NATIVE = add bare metal support
PPC_IBM_CELL_BLADE = add blade device drivers, etc.

Also renames spu_priv1.c to spu_priv1_mmio.c.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>