net: sparx5: Refactor mdb handling according to feedback

- Remove mact_lookup and use new mact_find instead.
- Make pgid_read_mask function.
- Set PGID arbiter to start searching at PGID_BASE + 8.

This is according to feedback on previous patch.
https://lore.kernel.org/netdev/20220322081823.wqbx7vud4q7qtjuq@wse-c0155/T/#t

Signed-off-by: Casper Andersson <casper.casan@gmail.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>