dt-bindings: some changes to match mt7988 dts(i) and driver

Documentation/devicetree/bindings/pinctrl/mediatek,mt7988-pinctrl.example.dtb: pinctrl@1001f000: compatible: ['mediatek,mt7988-pinctrl', 'syscon'] is too long
	from schema $id: http://devicetree.org/schemas/pinctrl/mediatek,mt7988-pinctrl.yaml#
Documentation/devicetree/bindings/pinctrl/mediatek,mt7988-pinctrl.example.dtb: pinctrl@1001f000: mdio0-pins:conf: 'groups' does not match any of the regexes: 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/pinctrl/mediatek,mt7988-pinctrl.yaml#
Documentation/devicetree/bindings/pinctrl/mediatek,mt7988-pinctrl.example.dtb: pinctrl@1001f000: mdio0-pins:conf: 'pins' is a required property
	from schema $id: http://devicetree.org/schemas/pinctrl/mediatek,mt7988-pinctrl.yaml#
Documentation/devicetree/bindings/pinctrl/mediatek,mt7988-pinctrl.example.dtb: pinctrl@1001f000: 'i2c0-pins-g0', 'mmc0-pins-emmc-51', 'mmc0-pins-sdcard' do not match any of the regexes: '-pins$', 'pinctrl-[0-9]+'

Author: frank-w