ARM: mxs: enable pinctrl dummy states

The mxs pinctrl driver will only support DT probe.  That said, the mxs
device drivers can only get pinctrl state from pinctrl subsystem when
the drivers get probed from device tree.

Before converting the whole mxs platform support over to device tree,
we need to enable pinctrl dummy states for those non-DT board files
to ensure the pinctrl API adopted by mxs device drivers will work for
both DT and non-DT probe.

Instead of calling pinctrl_provide_dummies() directly in every board
file, the patch introduces soc specific calls mx23_soc_init() and
mx28_soc_init() for boards' .init_machine hook to invoke, so that
any soc specific setup for non-DT boot only can be added there.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>