net: phy: mscc: report and configure in-band auto-negotiation for SGMII/QSGMII

The following Vitesse/Microsemi/Microchip PHYs, among those supported by
this driver, have the host interface configurable as SGMII or QSGMII:
- VSC8504
- VSC8514
- VSC8552
- VSC8562
- VSC8572
- VSC8574
- VSC8575
- VSC8582
- VSC8584

All these PHYs are documented to have bit 7 of "MAC SerDes PCS Control"
as "MAC SerDes ANEG enable".

Out of these, I could test the VSC8514 quad PHY in QSGMII. This works
both with the in-band autoneg on and off, on the NXP LS1028A-RDB and
T1040-RDB boards.

Notably, the bit is sticky (survives soft resets), so giving Linux the
tools to read and modify this settings makes it robust to changes made
to it by previous boot layers (U-Boot).

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250813074454.63224-1-vladimir.oltean@nxp.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>