gpio: mxc: add generic gpio request/free callbacks to pinctrl

If a GPIO controller description in board DTB contains information
about mappings between GPIOs and pads under IOMUX control use it to
request and free GPIOs with respect to pinctrl/pinmux subsystems.

One of immediate positive functional changes is inability to
request non-existing GPIOs, i.e. if there is no pad such. Also
pinctrl/pinmux may now properly account pads occupied by requested
GPIOs.

The change has no effect, if "gpio-ranges" property is not found
including the case if a board has no DTB firmware.

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>