ARM: dts: imx6: RIoTboard provide gpio-line-names

gpio-line-names may help to make work with GPIOs from user space easier.
Following examples are provided with libgpiod
https://github.com/brgl/libgpiod

 :
|# Toggle a GPIO by name, then wait for the user to press ENTER.
|$ gpioset --mode=wait `gpiofind "USR-LED-2"`=1
|# Pause execution until a single event of any type occurs. Don't print
|# anything. Find the line by name.
|$ gpiomon --num-events=1 --silent `gpiofind "USR-IN"`

Used names was taken from RIoTboard schematics, version 1 (2013.12.07).

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>