ARM: dts: imx6dl-colibri: Add gpio-line-names

Add GPIO line names on module level. Those are all GPIOs which a user
might use on his custom carrier board. If more meaningful names are
available on the carrier board, the user can overwrite the line names
in the carrier board level device tree.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>