dma: mmp_pdma: Staticize mmp_pdma_alloc_descriptor()

mmp_pdma_alloc_descriptor() is used only in this file.
Fix the following sparse warning:

drivers/dma/mmp_pdma.c:359:25: warning: symbol 'mmp_pdma_alloc_descriptor' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>