dmaengine: add dma_slave_get_caps api

add new device callback .device_slave_caps api which can be used by clients to
query the dma channel capablties before they program the channel. This can help
is removing errors during the channel programming. Also add helper
dma_slave_get_caps API

This patch folds the work done by Matt earlier
https://patchwork.kernel.org/patch/2094891/



Signed-off-by: Vinod Koul <vinod.koul@intel.com>