dmaengine: at_xdmac: Declare slave capabilities for the generic code

Now that the generic slave caps code can make use of the device assigned
capabilities, instead of relying on a callback to be implemented.

Make use of this code.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>