ARM: dts: at91: sama5d2 Xplained: use DMA for UART3

Use DMA for UART3 as we have enough channels and to show how to
specify DMA use with serial nodes.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

arch/arm/boot/dts/at91-sama5d2_xplained.dts

@@ -256,6 +256,8 @@ watchdog@f8048040 {
 			};
 
 			uart3: serial@fc008000 {
+				atmel,use-dma-rx;
+				atmel,use-dma-tx;
 				pinctrl-names = "default";
 				pinctrl-0 = <&pinctrl_uart3_default>;
 				status = "okay";