Resources need to be shared between pinmux and plgpio.

Introduce regmap phandle in order to retrieve the regmap
from the phandle if the property is present.
This allows to retrieve an external regmap (ie the one
used by pinmux if the phandle references the pinmux node)
from plgpio.
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20211202095255.165797-4-herve.codina@bootlin.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>