ARM: dts: at91: Enable DMA on sama5d4_xplained console

Enable DMA on usart3 to get a more reliable console. This is especially
useful for automation and kernelci were a kernel with PROVE_LOCKING enabled
is quite susceptible to character loss, resulting in tests failure.

Cc: stable <stable@vger.kernel.org> #v4.1+
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

arch/arm/boot/dts/at91-sama5d4_xplained.dts

@@ -116,6 +116,8 @@ slot@0 {
 			};
 
 			usart3: serial@fc00c000 {
+				atmel,use-dma-rx;
+				atmel,use-dma-tx;
 				status = "okay";
 			};