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>

arch/arm64/boot/dts/freescale/imx8mm-ddr4-evk.dts

@@ -23,7 +23,6 @@ status {
 &gpmi {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_gpmi_nand>;
-	nand-on-flash-bbt;
 	status = "okay";
 };
 

arch/arm64/boot/dts/freescale/imx8mn-bsh-smm-s2.dts

@@ -21,7 +21,6 @@ memory@40000000 {
 &gpmi {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_gpmi_nand>;
-	nand-on-flash-bbt;
 	status = "okay";
 };