spi: sirf: move to use generic dma dt-binding

sirf-dma driver enabled generic dt binding for dma channels.
see here we remove self-defined dma channel prop and move to
use generic dma_request_slave_channel.
related changes in dts is something like:
    dmas = <&dmac1 9>,
    <&dmac1 4>;
    dma-names = "rx", "tx";

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@linaro.org>