arm64: dts: imx8m: Remove 'nand-on-flash-bbt' from nand controller

The 'nand-on-flash-bbt' property is a property for the NAND device,
not for the GPMI nand controller.

Remove it to fix the following schema warnings:

imx8mm-ddr4-evk.dtb: nand-controller@33002000: Unevaluated properties are not allowed ('nand-on-flash-bbt' was unexpected)
	from schema $id: http://devicetree.org/schemas/mtd/gpmi-nand.yaml#
imx8mn-bsh-smm-s2.dtb: nand-controller@33002000: Unevaluated properties are not allowed ('nand-on-flash-bbt' was unexpected)
	from schema $id: http://devicetree.org/schemas/mtd/gpmi-nand.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>