pinctrl: Fix some typos and grammar issues in the documentation

I had been trying to learn a bit more about the pinctrl subsystem, and I
realized several typos and grammar issues while going through the documentation.
I have probably not caught all the possible issues, but this change is
addressing several places for improvement.

Signed-off-by: Laszlo Papp <lpapp@kde.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>