arm64: dts: marvell: explicitly enable the mdio nodes for 7k/8k DB

Explicitly enable the MDIO nodes in the Marvell Armada 7k DB and Marvell
Armada 8k DB. This is needed as the MDIO nodes will be disabled in the
CP 110 slave and master dtsi by a following up patch.

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