pinctrl: mxs: Make PINCTRL_MXS select PINMUX && PINCONF

Then we can remove "select PINMUX && PINCONF" from PINCTRL_IMX{23,28}.
This simplifies the dependency.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>