ARM: at91/DT: remove atmel,use-dma-* from 9x5 and 9n12 USART nodes

Fix the use of USART on both at91sam9x5 and at91sam9n12. In DTS, the
atmel,use-dma-[rx|tx] property is present but a DMA channel cannot be used.
Indeed the connexion between the DMA engine and the slave is not implemented
yet in Device Tree.
Note however that this property is also used for PDC (private DMA) on older
SoCs. This is why the driver alone cannot determine the validity of this
property.

Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>