pinctrl: pinconf-generic: add drive strength parameter

Some pin configurations IP allows to set the current output to the pin.
This patch adds such a parameter to the pinconf-generic mechanism.

This parameter takes as argument the drive strength in mA.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>