xtensa: implement dma_to_phys and phys_to_dma

This fixes the following build error seen in -next:

  drivers/gpu/drm/nouveau/nvkm/subdev/instmem/gk20a.c:143:2: error:
  implicit declaration of function 'dma_to_phys'

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>