ARM: dts: turris-omnia: add comphy handle to eth2

The eth2 controller on Turris Omnia is connected to SerDes. For SFP to
be able to switch between 1G and 2.5G modes the comphy link has to be
defined.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: f3a6a9f3 ("ARM: dts: add description for Armada 38x ...")
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Uwe Kleine-König <uwe@kleine-koenig.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>