pinctrl: ralink: add a pinctrl driver for the rt2880 family

These Socs have 1-3 banks of 8-32 gpios. Rather then setting the muxing of each
pin individually, these socs have mux groups that when set will effect 1-N pins.
Pin groups have a 2, 4 or 8 different muxes.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20201208075523.7060-3-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>