powerpc/cell: Extract duplicated IOPTE_* to <asm/iommu.h>

Both arch/powerpc/platforms/cell/iommu.c and arch/powerpc/platforms/ps3/mm.c
contain the same Cell IOMMU page table entry definitions. Extract them and move
them to <asm/iommu.h>, while adding a CBE_ prefix.
This also allows them to be used by drivers.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>