ARM: dts: imx6-tx6: remove LED pinctrl setting from hoggrp

Move the pinctrl setting for the board LED from the hoggrp node
to a separate node referenced by the LED driver, so that the pin is
free to be used for different purpose when the LED driver is disabled.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>