arm64: dts: marvell: add xmdio nodes for 7k/8k

Add the description of the xMDIO bus for the Marvell Armada 7k and
Marvell Armada 8k; for both CP110 slave and master. This bus is found
on Marvell Ethernet controllers and provides an interface with the
xMDIO bus.

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