gpio: mxc: shift gpio_mxc_init() to subsys_initcall level

In general situation on-SoC GPIO controller drivers should be probed
after pinctrl/pinmux controller driver, because on-SoC GPIOs utilize a
pin/pad as a resource provided and controlled by pinctrl subsystem.

This is stated in multiple places, e.g. from drivers/Makefile:

  GPIO must come after pinctrl as gpios may need to mux pins etc

Looking at Freescale iMX SoC series specifics, imx*_pinctrl_init()
functions are called at arch_initcall and postcore_initcall init
levels, so the change of initcall level for gpio-mxc driver from
postcore_initcall to subsys_initcall level is sufficient. Also note
that the most of GPIO controller drivers settled at subsys_initcall
level.

If pinctrl subsystem manages pads with GPIO functions, the change is
needed to avoid unwanted driver probe deferrals during kernel boot.

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>