Documentation: pinctrl: Add "pinmux" property

Document "pinmux" property as part of generic pin controller
documentation.
Fix 2 minor typos in documentation while at there.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>