spi: omap2-mcspi: add generic DMA request support to the DT binding

The binding definition is based on the generic DMA request binding

Signed-off-by: Matt Porter <mporter@ti.com>
Signed-off-by: Joel A Fernandes <joelagnel@ti.com>
Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>