MIPS: pci-ar2315: include <linux/dma-direct.h> for phys_to_dma

Ensure this file has a prototype for phys_to_dma and dma_to_phys.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>