pinctrl: mxs: do not export mxs_pinctrl_probe

No user of mxs_pinctrl_probe() can be built as a module, hence
exporting the symbol is not necessary. Drop EXPORT_SYMBOL_GPL.

Inspired by a patch from Stefan Agner for the tegra pinctrl driver.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>