spi: sirf: use DMA if both buffer address and length are aligned

this patch enables DMA support for SiRFSoC SPI driver, if both
buffers and length are aligned with DMA controller's hardware
limitation, use generic SiRF generic dmaengine driver.

for PIO, SiRF SPI controller actually is using rx to trigger rx,
that means if we write any word to tx fifo, we will get a word
from rx fifo. for DMA, we use two different channel for tx and
rx, and issue them both for every transfer.
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@linaro.org>