arm64: Enable dma_mmap_nonconsistent

Now that the non-consistent behavior is fully integrated into the
dma mapping subsystem, allow user space to mmap nonconsistent dma
buffers.

Change-Id: I1b95ba9a1751e00a5812d9df582af20f316fcafe
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>