ARM: dts: mxs: add comments for pinmux-ids

Add comments for pinmux-ids so that we do not have to keep looking at
Documentation/devicetree/bindings/pinctrl/fsl,mxs-pinctrl.txt to read
all these pinmux-ids.

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