pinctrl: microchip-sgpio: update to support regmap

Adopt regmap instead of a direct memory map so that custom regmaps and
other interfaces can be supported.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/20211119195928.2498441-5-colin.foster@in-advantage.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>