ARM: dts: mxs: add pinctrl header files

Remove the list of possible pin configurations from the documentation
file and create header files containing those definitions.
This eliminates the need for error-prone manual lookup of those values
in the documentation and guarantees consistency between the human
readable representation of the pad function in the .dts file and the
actual binary value used in DT.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>