pinctrl: pinctrl-microchip-sgpio: Add pinctrl driver for Microsemi Serial GPIO

This adds a pinctrl driver for the Microsemi/Microchip Serial GPIO
(SGPIO) device used in various SoC's.

The driver is added as a pinctrl driver, albeit only having just GPIO
support currently. The hardware supports other functions that will be
added following.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Link: https://lore.kernel.org/r/20201113145151.68900-3-lars.povlsen@microchip.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>