dmaengine: sprd: statify 'sprd_dma_prep_dma_memcpy'

Sparse warns that 'sprd_dma_prep_dma_memcpy' should be static so make it
static.

drivers/dma/sprd-dma.c:713:32: warning:
symbol'sprd_dma_prep_dma_memcpy' was not declared. Should it be static?

Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>