pinctrl: freescale: imx: implement gpio_disable_free for Vybrid

The Freescale Vybrid SoC has GPIO capabilities as part of the
IOMUXC. To enable GPIO's, the gpio_request_enable callback has
been implemented, however the corsponding gpio_disable_free
callback is missing. So far, disabling (unexporting) a GPIO left
the pin in its last state.

Implement a proper gpio_disable_free function which clears the
three enable bits which influence the state (IBE, OBE and PUE).

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>