We rely on devices to use pinmuxing configurations in DT to select the
GPIO function (function 0) if they're going to use the gpio in GPIO
mode. Let's simplify things for driver authors by implementing
gpio_request_enable() for this pinctrl driver to mux out the GPIO
function when the gpio is use from gpiolib.

Cc: Doug Anderson <dianders@chromium.org>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>