drivers: spi: sam0: Implement DMA async API

This adds support for the async API for SAM0 SERCOM SPI using
DMA to drive the device.  This implementation does the reload
for both transmit and receive in the receive DMA handler.
Doing this simplifies the implementation but means that the
transmit drains completely, resulting in the SPI clock pausing
between buffers while both are reloaded in the receive handler.

Tested with tests/drivers/spi/spi_loopback and several simple
programs monitored with a logic analyzer.

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