net: ethernet: mtk_eth_soc: add support for MT7988 internal 2.5G PHY

net: ethernet: mtk_eth_soc: add support for MT7988 internal 2.5G PHY

The MediaTek MT7988 SoC comes with an single built-in Ethernet PHY for
2500Base-T/1000Base-T/100Base-TX/10Base-T link partners in addition to
the built-in 1GE switch. The built-in PHY only supports full duplex.

Add muxes allowing to select GMAC2->2.5G PHY path and add basic support
for XGMAC as the built-in 2.5G PHY is internally connected via XGMII.
The XGMAC features will also be used by 5GBase-R, 10GBase-R and USXGMII
SerDes modes which are going to be added once support for standalone PCS
drivers is in place.

In order to make use of the built-in 2.5G PHY the appropriate PHY driver
as well as (proprietary) PHY firmware has to be present as well.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/9072cefbff6db969720672ec98ed5cef65e8218c.1745715380.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>