pinctrl: ralink: move MT7621 SoC pinmux config into a new 'pinctrl-mt7621.c' file

Move all related code for SoC MT7621 into a new driver located
in 'pinctrl-mt7621.c' source file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210604115159.8834-3-sergio.paracuellos@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>