powerpc: move device_to_mask() to dma-mapping.h

Move device_to_mask() to dma-mapping.h because we need to use it from
outside dma_64.c in a later patch.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>