powerpc/cell: Move CBE_IOPTE_* to <asm/cell-regs.h>

As <asm/iommu.h> doesn't contain any other hardware specific definitions
but only interfaces.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>