dt-bindings: pinctrl: mediatek: mt7620: split binding

The MT7628 and MT7688 SoCs contain different pin muxing information,
therefore, should be split. This can be done now that there are compatible
strings to distinguish them from other SoCs.

Split the schema out to mediatek,mt76x8-pinctrl.yaml.

Remove mediatek,mt76x8-pinctrl from mt7620.
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230317213011.13656-21-arinc.unal@arinc9.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>