spi: davinci: enable DMA when channels are defined in DT

When booting with DT the SPI driver is always using
the SPI_IO_TYPE_INTR mode to transfer data even if DMA channels are
defined in the DT.

This commit changes the behaviour to select the SPI_IO_TYPE_DMA mode
if DMA channels are defined in the DT and will keep SPI_IO_TYPE_INTR
if the channels are not defined in it.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Frode Isaksen <fisaksen@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>