pinctrl: spear: plgpio: Convert to regmap

Resources need to be shared between pinmux and plgpio.

Use regmap (syscon) to access resources to allow an
easy way to share resources.
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20211202095255.165797-3-herve.codina@bootlin.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>