pinctrl: ralink: mt7620: split out to mt76x8

pinctrl: ralink: mt7620: split out to mt76x8

Split the driver out to pinctrl-mt76x8.c. Remove including the unnecessary
headers since is_mt76x8() is not being used anymore.

Introduce a new compatible string to be able to document the pin muxing
information properly.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20230317213011.13656-4-arinc.unal@arinc9.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>