ARM: at91: sama5d3xek: reserve dma channel for audio

We set the DMA configuration on USARTs in the SoC DT in (ARM: at91: sama5d3:
add usart dma configurations). As the audio must work with DMA channels, we
reserve some dma channels for audio, or else audio won't work.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
[nicolas.ferre@atmel.com: move to the sama5d3xmb.dtsi to cover all board variants]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>