pinctrl: add a pincontrol driver for BCM6318

pinctrl: add a pincontrol driver for BCM6318

Add a pincontrol driver for BCM6318. BCM6318 allows muxing most GPIOs
to different functions. BCM6318 is similar to BCM6328 with the addition
of a pad register, and the GPIO meaning of the mux register changes
based on the GPIO number.

Co-developed-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Link: https://lore.kernel.org/r/20210324081923.20379-23-noltari@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>