ARM: dts: imx6ul: Fix nand-controller #size-cells

nand-controller.yaml bindings says #size-cells shall be set to 0.
Fixes the dtbs_check warning:
arch/arm/boot/dts/nxp/imx/imx6ul-tqma6ul2l-mba6ulx.dtb:
 nand-controller@1806000: #size-cells:0:0: 0 was expected
 from schema $id: http://devicetree.org/schemas/mtd/gpmi-nand.yaml#



Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>