drivers: i2c: i2c_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-i2c binding for dma to replace the dma property with
proper 'dmas' property.

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