fixup: restore dma-mapping.h

dwc_otg has been fixed, so no need to revert 6ce0d200:
ARM: dma: Use dma_pfn_offset for dma address translation

The pfn_to_dma/dma_to_pfn changes that came with that commit
is needed to use the 'dma-ranges' DT property on ARCH_BCM2835.
dma-ranges is needed by bcm2708_fb and vchiq on ARCH_BCM2835.
If not the mailbox call fails to hand over the correct
bus address to videocore.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>