net: ethernet: mtk_eth_soc: add basic support for MT7988 SoC

Introduce support for ethernet chip available in MT7988 SoC to
mtk_eth_soc driver. As a first step support only the first GMAC which
is hard-wired to the internal DSA switch having 4 built-in gigabit
Ethernet PHYs.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/25c8377095b95d186872eeda7aa055da83e8f0ca.1690246605.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>