net: mvpp2: phylink support

Convert the PPv2 driver to implement phylink helpers, and use phylink in
DT mode. The other mode supported is ACPI, which will need further work
in order to be entirely compatible with phylink.

The MAC and GoP configuration functions were completely moved to fit
into the phylink helpers. When a PHY is always present between the MAC
and the physical port, phylink only is used, but when this is not the
case (the MAC directly is connected to the physical port) the link IRQ
is used to detect changes in the link state and call phylink_mac_change.

The ACPI mode do not uses phylink as of now, and the changes shouldn't
impact its use.

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