net: ethernet: mtk_eth_soc: add basic XDP support

Introduce basic XDP support to mtk_eth_soc driver.
Supported XDP verdicts:
- XDP_PASS
- XDP_DROP
- XDP_REDIRECT

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>