ARM: dts: sunxi: Conform to DT spec for NAND controller

The NAND controller node name should be nand-controller and not nand as we
used previously according to the devicetree specification. Let's fix our
DTs.

Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>