ARM: dts: turris-omnia: Add label for wan port

Device tree label property should contain label from the box/stick.
Labels for other ports are already specified in DT but wan is missing.
So add missing label for wan port.

Fixes: 26ca8b52 ("ARM: dts: add support for Turris Omnia")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>