gpio/pinctrl: imx: let IOMUX controller know about on-SoC GPIOs

The change establishes a connection between on-SoC IOMUX controller(s) and GPIO controllers found on some SoC from Freescale/NXP iMX series, if a GPIO controller device node contains common gpio-ranges information.

The change is backward compatible with respect to potentially not updated outdated DTB data without gpio-ranges propery, for such boards the only functional change is lowered initcall priority of GPIO controller driver, which in general anyway is exected to be used only after pinctrl/pinmux controller.

If this change is applied the next interesting applications may be done as a follow-up work, for example switching pad function to GPIO on gpiod request, converting iomux controller driver to strict type and so on.

For actual values of gpio-ranges properties please reference series "ARM: dts: imx: add gpio-ranges properties to some iMX GPIO controllers"
http://www.spinics.net/lists/arm-kernel/msg525258.html

Changes from v1 to v2:
* replaced 2/3 by an own change providing a better commit description and which moves gpio_mxc_init() call to subsys_initcall() instead of apparently too late device_initcall(), this mitigates Shawn's expressed concern about the change.

Vladimir Zapolskiy (3):
pinctrl: imx: accept gpio request/free from pinctrl
gpio: mxc: shift gpio_mxc_init() to subsys_initcall level
gpio: mxc: add generic gpio request/free callbacks to pinctrl