dt-bindings: mtd: atmel-nand: add sam9x60 compatible

Add compatibility string for the sam9x60 nand controller.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>