net: mvpp2: finally add the PPv2.2 compatible string

Now that the mvpp2 driver has been modified to accommodate the support
for PPv2.2, we can finally advertise this support by adding the
appropriate compatible string.

At the same time, we update the Kconfig description of the MVPP2 driver.

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