ARM: dts: microchip: at91sam9x5ek: Use DMA for DBGU serial port

The UART controller does not seem to be fast enough to receive data (at
115200 baud) without dropping bytes when DMA is disabled.

This has often been noticed when pasting SSH keys into the serial
console (ttyS0). Only after multiple tries, the long strings would be
successfully transferred without missing characters.

Co-developed-by: Michael Zimmermann <[e-mail]>
Signed-off-by: Michael Zimmermann <[e-mail]>
Signed-off-by: Ezra Buehler <[e-mail]>
Acked-by: Nicolas Ferre <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[claudiu.beznea: s/Co-Developed-by/Co-developed-by to make checkpatch.pl
happy, s/at91/microchip in commit title]
Signed-off-by: Claudiu Beznea <[e-mail]>

arch/arm/boot/dts/microchip/at91sam9x5ek.dtsi

···
 };
 
 &dbgu {
+	atmel,use-dma-rx;
+	atmel,use-dma-tx;
 	status = "okay";
 };