dmaengine: mmp_tdma: statify symbols

Sparse complains:

drivers/dma/mmp_tdma.c:407:22: warning: symbol 'mmp_tdma_alloc_descriptor' was not declared. Should it be static?
drivers/dma/mmp_tdma.c:595:17: warning: symbol 'mmp_tdma_xlate' was not declared. Should it be static?

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Cc: Qiao Zhou <zhouqiao@marvell.com>