pinctrl: Add pincontrol driver for ARTPEC-6 SoC

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: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>