arm64: dts: marvell: 7040-db: enable the SFP port

This patch enables the SFP port on the Armada 7040 DB as this port
is now supported by the PPv2 driver (since the PHY is now optional).

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>