net: mvpp2: Fix clock resource by adding missing mg_core_clk

Marvell's PPv2.2 IP needs an additional clock named "MG Core clock".
This is required on Armada 7K and 8K.

This commit adds the required clock in mvpp2, making sure it's only
used on PPv2.2.

Fixes: c7e92def

 ("clk: mvebu: cp110: Fix clock tree representation")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>