dmaengine: at_hdmac: declare slave capabilities

Declare slave capabilities to suppress "this driver doesn't support generic
slave capabilities reporting" warning.

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