arm64: dts: imx8mq-zii-ultra: Fix mdio node name

Per mdio-gpio.yaml, the node name should be 'mdio'.

Change it accordingly to fix the following schema warning:

imx8mq-zii-ultra-zest.dtb: bitbang-mdio: $nodename:0: 'bitbang-mdio' does not match '^mdio(@.*)?'
	from schema $id: http://devicetree.org/schemas/net/mdio-gpio.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mq-zii-ultra.dtsi

@@ -15,7 +15,7 @@ chosen {
 		stdout-path = &uart1;
 	};
 
-	mdio0: bitbang-mdio {
+	mdio0: mdio {
 		compatible = "virtual,mdio-gpio";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_mdio_bitbang>, <&pinctrl_fec1_phy_reset>;