dmaengine: mmp_tdma: Fill in slave capabilities

This makes dma_get_slave_caps() work with the device so that it could
actually be used with soc-generic-dmaengine-pcm.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200419164912.670973-7-lkundrak@v3.sk


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