ARM/dma-mapping: use the generic versions of dma_to_phys/phys_to_dma by default

Only the footbridge platforms provide their own DMA address translation
helpers, so switch to the generic version for all other platforms, and
consolidate the footbridge implementation to remove two levels of
indirection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Marc Zyngier <maz@kernel.org>