arm64: dts: marvell: mcbin: enable the mdio node

Since the mdio nodes are disabled by default now, we should explicitly
enable these nodes at the board level when they are used. Enable the
cpm_mdio node for the 8040-mcbin.

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