uart_sam0: extend async serial tests to more atsamxxx_xpro boards.

This includes new overlays for boards supporting DMA, and one more test
to verify M0 boards both with and without DMA can be built when
requesting CONFIG_UART_ASYNC_API.

Signed-off-by: Diego Elio Pettenò <flameeyes@meta.com>
Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>