net: sparx5: redefine internal ports and PGID's as offsets

Internal ports and PGID's are both defined relative to the number of
front ports on Sparx5. This will not work on lan969x. Instead make them
offsets to the number of front ports and add two helpers to retrieve
them. Use the helpers throughout.

Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>