pinctrl: imx: update fsl,pins description in bindings doc

While commit e1641531 (pinctrl: imx: move hard-coding data into device
tree) moves to use DTC macro for imx pinctrl device tree setting, it
changes the semantics of fsl,pins without updating the bindings doc
properly.  Let's update the fsl,pins description to stop confusing
people.

While at it, the example in the document is updated, and the stale TODO
gets removed.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>