drivers: dma: sam0: Implement get_status API

Implement the get_status DMA API for the SAM0 DMA controller.
The busy field is set only when the selected channel is
actively transferring data (i.e. both enabled and selected by the
arbiter).  The direction field is left unset, since that
information is not normally retained by the DMA controller.

Signed-off-by: Derek Hageman <hageman@inthat.cloud>