drivers: spi: spi_sam0: rework devicetree support

Rework the devicetree to utilize new DT_INST macros and extract per
instance data for clocks and dma from devicetree.  We update the
atmel,sam0-spi binding for dma to replace the rxdma and txdma
properties with proper 'dmas' property.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>