drivers: dma: Add SAM0 DMAC driver

This adds support for the SAM0 DMA Controller (DMAC).  Chained
transfer are not currently implemented.

Tested with tests/drivers/dma/loop_transfer and custom modifications
to that test using three parallel reloading channels.  Also tested
with a trivial program that did memory->serial.

Signed-off-by: Michael Hope <mlhx@google.com>

[hageman@inthat.cloud: Rebased and updated commit message]
Signed-off-by: Derek Hageman <hageman@inthat.cloud>