pinctrl: mxs: register driver at postcore_initcall time

It's not so usual, but there are still some cases which require pinctrl
driver function at arch_initcall time.  So register imx23 and imx28
pinctrl driver at postcore_initcall time.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>