pinctrl: generic: Add output-enable property

Add output-enable generic pin configuration property.
This properties allows enabling/disabling pin's output capabilities
without actually driving any value on the line.

Acked-by: Rob Herring <robh@kernel.org>
[Added inline elaborations on buffer enabling/disabling]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>