pinctrl: pinctrl-mxs: document the missing pull-ups

Some pins on the i.mx23 and i.mx28 are missing pull-ups, document that
oddity because it was difficult to know the expected behaviour.
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

Documentation/devicetree/bindings/pinctrl/fsl,mxs-pinctrl.txt

@@ -70,6 +70,10 @@ Optional subnode-properties:
     0: Disable the internal pull-up
     1: Enable the internal pull-up
 
+Note that when enabling the pull-up, the internal pad keeper gets disabled.
+Also, some pins doesn't have a pull up, in that case, setting the fsl,pull-up
+will only disable the internal pad keeper.
+
 Examples:
 
 pinctrl@80018000 {