spi: imx: support DMA for imx35

Support DMA transfers on imx35 and compatible chipsets (imx31, imx25).

If DMA can be used, set the start mode control (SMC) bit to start the
SPI burst as soon as data is written into the tx fifo. Configure DMA
requests when the fifo is half empty during tx or half full during rx.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Mark Brown <broonie@kernel.org>