dt-bindings: pinctrl: mt7620: add proper function muxing binding

Not every function can be muxed to a group. Add proper binding which
documents which function can be muxed to a group or set of groups.
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20221231160849.40544-3-arinc.unal@arinc9.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>