arm64: dma-mapping: make dma_ops const

dma_ops should be const since they shouldn't be changing after being
defined (there's even a checkpatch warning for this). Convert existing
dma_ops to const.

Change-Id: Ic27ba9a0832b12cae43cd45c72f053335845fd8f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>