ARM/dma-mapping: use dma_to_phys/phys_to_dma in the dma-mapping code

Use the helpers as expected by the dma-direct code in the old arm
dma-mapping code to ease a gradual switch to the common DMA code.

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