MIPS: Add missing calls to plat_unmap_dma_mem.

dma_free_noncoherent() and dma_free_coherent() are missing calls to
plat_unmap_dma_mem().  This patch adds them.

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