dma: omap-dma: Implement device_slave_caps callback

With the callback implemented omap-dma can provide information to client
drivers regarding to supported address widths, directions, residue
granularity, etc.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>