pinctrl: imx: accept gpio request/free from pinctrl

While only Freescale Vybrid SoC has settings of GPIO capabilities done
by iomux controller, it is only a matter of GPIO controller driver
implementation for the rest of Freescale/NXP SoCs from iMX series.

As a practical example on GPIO request a pad function should be
switched to GPIO, but because this requires updates to all particular
iMX pinctrl drivers, for simplicity at the moment add only a proper
connection between shared pinctrl-imx and pinctrl/pinmux core, namely
.gpio_request_enable/.gpio_disable_free/.gpio_set_direction callbacks
should return success to a caller. This change allows to progress by
adding request/free callbacks into gpio-mxc.c driver.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>