dt-bindings: mfd: atmel-usart: add DMA bindings for USART in SPI mode

The bindings for DMA are now common for both drivers of the USART
IP.

The node given as an example for USART in SPI mode has been updated in
order to include DMA bindings.

Signed-off-by: Radu Pirea <radu_nicolae.pirea@upb.ro>
Signed-off-by: Mark Brown <broonie@kernel.org>