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>