dt-bindings: net: mediatek,net: add mt7988-eth binding

dt-bindings: net: mediatek,net: add mt7988-eth binding

Introduce DT bindings for the MT7988 SoC to mediatek,net.yaml.
The MT7988 SoC got 3 Ethernet MACs operating at a maximum of
10 Gigabit/sec supported by 2 packet processor engines for
offloading tasks.
The first MAC is hard-wired to a built-in switch which exposes
four 1000Base-T PHYs as user ports.
It also comes with built-in 2500Base-T PHY which can be used
with the 2nd GMAC.
The 2nd and 3rd GMAC can be connected to external PHYs or provide
SFP(+) cages attached via SGMII, 1000Base-X, 2500Base-X, USXGMII,
5GBase-KR or 10GBase-KR.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/3c83d2c0d629dac064ec4396132538c52e77a57f.1690246066.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>