net: mscc: ocelot: introduce macros for iterating over PGIDs

The current iterators are impossible to understand at first glance
without switching back and forth between the definitions and their
actual use in the for loops.

So introduce some convenience names to help readability.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>