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 <[email protected]>

Author: ElectroInnovator

drivers/dma/dma_sam0.c

@@ -413,6 +413,13 @@ static int dma_sam0_init(const struct device *dev)
 	PM->APBBMASK.bit.DMAC_ = 1;
 #endif
 
+	/* Reset the DMA controller */
+	DMAC->CTRL.bit.DMAENABLE = 0;
+	DMAC->CTRL.bit.CRCENABLE = 0;
+	DMAC->CTRL.bit.SWRST = 1;
+	while (DMAC->CTRL.bit.SWRST) {
+	}
+
 	/* Set up the descriptor and write back addresses */
 	DMA_REGS->BASEADDR.reg = (uintptr_t)&data->descriptors;
 	DMA_REGS->WRBADDR.reg = (uintptr_t)&data->descriptors_wb;