pinctrl: rp1: Implement RaspberryPi RP1 pinmux/pinconf support

The current implementation for the pin controller peripheral
on the RP1 chipset supports gpio functionality and just the
basic configuration of pin hw capabilities.

Add support for selecting the pin alternate function (pinmux)
and full configuration of the pin (pinconf).

Related pins are also gathered into groups.

Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/all/8c282b89b1aa8b9e3c00f6bd3980332c47d82df7.1750778806.git.andrea.porta@suse.com/
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>

Author: Andrea della Porta