dmaengine: mmp_pdma: Allow building as a module

There is no reason the Marvell MMP peripheral DMA driver would have
to be built-in.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121110356.1768635-3-lkundrak@v3.sk


Signed-off-by: Vinod Koul <vkoul@kernel.org>