ARM: mmp: select PINCTRL for ARCH_MMP

This makes PINCTRL related config options visible.
Otherwise there is no way to build pinctrl drivers for MMP2, PXA168 and PXA910.

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