arm64: dts: armada-3720-turris-mox: update ethernet-phy handle name

Use property name `phy-handle` instead of the deprecated `phy` to
connect eth2 to the PHY.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 7109d817 ("arm64: dts: marvell: add DTS for Turris Mox")
Cc: Gregory CLEMENT <gregory.clement@bootlin.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>