ARM: at91/DT: add NAND + DMA property

Add the "atmel,nand-has-dma" property to NAND node for SoC that
can use the DMA to perform NAND accesses.
Use of this property was added in 1b719265
(mtd: atmel_nand: add a new dt binding item for nand dma support).

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Boris BREZILLON <b.brezillon@overkiz.com>