net: mvpp2: update the port documentation regarding the GoP

The Marvell PPv2 port structure stores the GoP id of a given port. This
information is specific to PPv2.2, but cannot be used by PPv2.1. Update
its comment to denote this specificity.

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