pinctrl: indicate GPIO direction on single GPIO request

pinctrl: indicate GPIO direction on single GPIO request

When requesting a single GPIO pin to be muxed in, some controllers
will need to poke a different value into the control register
depending on whether the pin will be used for GPIO output or GPIO
input. So pass this info along for the gpio_request_enable()
function, we assume this is not needed for the gpio_free_disable()
function for the time being.

ChangeLog V1->V2:
- This also amends the documentation to make it clear the this
  function and associated machinery is *ONLY* intended as a backend
  to gpiolib machinery, not for everyone and his dog to start playing
  around with pins.

Suggested-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>