dmaengine: sirf: Add device_slave_caps interface

this patch adds device_slave_caps() callback as SiRF SoC sound drivers
depend on it.

Signed-off-by: Rongjun Ying <rongjun.ying@csr.com>
Signed-off-by: Barry Song <Barry.Song@csr.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>