net: mvpp2: initialize the comphy

On some platforms, the comphy is between the MAC GoP and the PHYs. The
mvpp2 driver currently relies on the firmware/bootloader to configure
the comphy. As a comphy driver was added to the generic PHY framework,
this patch uses it in the mvpp2 driver to configure the comphy at boot
time to avoid relying on the bootloader.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>