pinctrl: imx: drop redundant OF dependency

The IMX/MXC selects USE_OF at architecture level, so the OF dependency
at individual SoC pinctrl driver level can just be dropped.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>