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: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>