dma: mxs-dma: Export missing symbols from mxs-dma.c

mxs-dma.c provides two functions mxs_dma_is_apbh and mxs_dma_is_apbx
which are used at least in mxs-mmc.c. Building mxs-mmc as module
fails due to those two symbols not being exported.

Signed-off-by: Attila Kinali <attila@kinali.ch>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>