dmaengine: sirf: enable generic dt binding for dma channels

move to support of_dma_request_slave_channel() and dma_request_slave_channel.
we add a xlate() to let dma clients be able to find right dma_chan by generic
"dmas" properties in dts.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>