drivers: dma: sam0: Reset DMA during initialization.

Fixes issue #83555, where UART transmit operations fail in Zephyr
sysbuild projects using MCUboot and the asynchronous UART API
(`CONFIG_UART_ASYNC_API=y`) on SAM0 devices such as the
ATSAMC21G18A.

The issue occurs because the DMA controller is not reset during
initialization, causing `BASEADDR` and `WRBADDR` registers to retain
MCUboot's configuration. This prevents the application from reconfiguring
these registers to its own RAM addresses, leading to UART transmit
timeouts and triggering the `UART_TX_ABORTED` callback.

This patch resolves the issue by resetting the DMA controller during
initialization in `dma_sam0.c`. The following actions are performed:
- Disables the DMA and CRC modules.
- Applies a software reset to ensure a clean state for reconfiguration.

With this change, UART transmit operations work as expected, improving
stability and compatibility between MCUboot and the application.

Signed-off-by: Tristen Pierson <tpierson@electrohire.com>