net: ethernet: mtk_eth_soc: move gdma_to_ppe and ppe_base definitions in mtk register map

This is a preliminary patch to introduce mt7986 hw packet engine.

Tested-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>