arm64: dts: imx8mn: Fix NAND controller size-cells

The NAND controller size-cells should be 0 per DT bindings.
Fix the following warning produces by DT bindings check:
"
nand-controller@33002000: #size-cells:0:0: 0 was expected
nand-controller@33002000: Unevaluated properties are not allowed ('#address-cells', '#size-cells' were unexpected)
"

Fixes: 6c3debcb

 ("arm64: dts: freescale: Add i.MX8MN dtsi support")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>