driver: serial: silabs: implement asynch usart with dma

First implementation of asynchronous usart driver with dma support.

Signed-off-by: Martin Hoff <[email protected]>

Author: Martinhoff-maker

drivers/serial/Kconfig.silabs_usart

@@ -8,6 +8,9 @@ config UART_SILABS_USART
 	select SERIAL_HAS_DRIVER
 	select SERIAL_SUPPORT_INTERRUPT
 	select SOC_GECKO_USART
+	select SERIAL_SUPPORT_ASYNC \
+		if DT_HAS_SILABS_LDMA_ENABLED
+	select DMA if UART_ASYNC_API
 	select PINCTRL
 	select CLOCK_CONTROL
 	select PM_DEVICE if PM