ARM: at91: at91sam9x5 dt: add usart dma definitions to dt

This patch adds usart dma definitions to both dtsi for sam9x5 chips. Without
usage of dma it's unable to catch all bytes on usart receiver.

Signed-off-by: Jiri Prchal <jiri.prchal@aksignal.cz>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>