pinctrl: enable pinmux for mmp series

Support PXA168/PXA910/MMP2 pinmux. Now only support function switch.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
[Rebase and fix some whitespace issues]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>