arm64: dts: marvell: 7040-db: describe the 10G interface as fixed-link

This patch adds a fixed-link node to the 10G interface of the 7040-db
board. This is required as the mvpp2 driver now uses phylink. The best
solution would have been to describe the SFP cage but they are not
wired correctly, and thus unusable, so we chose to use fixed-link
instead.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>