drivers: dma: atmel_sam0: Convert to use devicetree macros for dma dev

Convert to using DT_INST_LABEL() in the dma driver and convert dma users
to use the DMA property macros to get the dma controller name.  We make
the assumption in the drivers that there is a single DMA controller
instance.

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