net: mvpp2: add AXI bridge initialization for PPv2.2

The PPv2.2 unit is connected to an AXI bus on Armada 7K/8K, so this
commit adds the necessary initialization of the AXI bridge.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>