MIPS: Don't unmap the memory for dma_sync*.

We were getting away with this for so long only because the only platform
with a non-empty plat_unmap_dma_mem() doesn't call dma_sync_sg_for_cpu()
and dma_sync_sg_for_device() from its commonly used drivers.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>