ARM: dts: colibri-imx6ull: correct wrong pinmuxing and add comments

Some pinmuxings are obviously wrong, originating from a copy/paste
error. This patch corrects that with the following strategy:

- Set all reserved bits to zero
- Leave drive strength and slew rate as is
- Add sensible pull and hysteresis depending on the function of the pin
- Not used pins are muxed to their reset-value defined by the SoC
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>