arm64: dts: marvell: 8040-db: describe the 10G interfaces as fixed-link

This patch adds a fixed-link node to both 10G interfaces of the 8040-db
board. This is required as the mvpp2 driver now uses phylink. The best
solution would have been to describe the SFP cages 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>