ARM: dts: turris-omnia: Add switch port 6 node

Switch port 6 is connected to eth0, so add appropriate device tree node for it.

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>