ARM: dts: at91: Enable DMA on sama5d2_xplained console

Enable DMA on uart1 to get a more reliable console.

Cc: stable <stable@vger.kernel.org> #v4.3+
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>