arm64: dts: marvell: cp110: add required clocks for mdio interface

Add the three required clocks for the MDIO interface to be functional
on Armada 8k platforms.  Without this, the CPU hangs, causing RCU
stalls or the system to become unresponsive.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
[Thomas:
 - remove mg_core_clock, since it's a parent of mg_clock
 - also add clock references to the slave CP mdio instance]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>