net: ethernet: mtk_eth_soc: ppe: add support for multiple PPEs

Add the missing pieces to allow multiple PPEs units, one for each GMAC.
mtk_gdm_config has been modified to work on targted mac ID,
the inner loop moved outside of the function to allow unrelated
operations like setting the MAC's PPE index.
Introduce a sanity check in flow_offload_replace to account for
non-MTK ingress devices.
Additional field 'ppe_idx' was added to struct mtk_mac in order
to keep track on the assigned PPE unit.

Signed-off-by: Elad Yifee <eladwf@gmail.com>
Link: https://lore.kernel.org/r/20240607082155.20021-1-eladwf@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>