Add pinctrl driver support for the Axis ARTPEC-6 SoC.
There are only some pins that actually have different
functions available, but all can control bias (pull-up/-down)
and drive strength.

Code originally written by Chris Paterson.
Signed-off-by: NJesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>