dmaengine: core: expose max burst capability to clients

This patch add max_burst to dma_get_slave_caps for clients
to get the burst capability of slave dma controller.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>