net: mvpp2: enable using phylink with ACPI

Now that the MDIO and phylink are supported in the ACPI
world, enable to use them in the mvpp2 driver. Ensure a backward
compatibility with the firmware whose ACPI description does
not contain the necessary elements for the proper phy handling
and fall back to relying on the link interrupts instead.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>