uart_sam0: don't consume CONFIG_UART_ASYNC_API directly.

There are situations in which the async API for uart can be provided
by another driver (case in point, uart_rtt), and thus there is no valid
DMA controller for the uart_sam0 driver to talk with.

By separating the configuration, there's no need to exclude samd20-based
boards (that have no DMA peripheral) from the uart_async_api tests.

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