net: mvpp2: add support for an additional clock needed for PPv2.2

The PPv2.2 variant of the network controller needs an additional
clock, the "MG clock" in order for the IP block to operate
properly. This commit adds support for this additional clock to the
driver, reworking as needed the error handling path.

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