net: phy: phy-c45: add SQI and SQI+ support for OATC14 10Base-T1S PHYs

net: phy: phy-c45: add SQI and SQI+ support for OATC14 10Base-T1S PHYs

Add support for reading Signal Quality Indicator (SQI) and enhanced SQI+
from OATC14 10Base-T1S PHYs.

- Introduce MDIO register definitions for DCQ_SQI and DCQ_SQIPLUS.
- Add `genphy_c45_oatc14_get_sqi_max()` to return the maximum supported
  SQI/SQI+ level.
- Add `genphy_c45_oatc14_get_sqi()` to return the current SQI or SQI+
  value.
- Update `include/linux/phy.h` to expose the new APIs.

SQI+ capability is read from the Advanced Diagnostic Features Capability
register (ADFCAP). If SQI+ is supported, the driver calculates the value
from the MSBs of the DCQ_SQIPLUS register; otherwise, it falls back to
basic SQI (0-7 levels). This enables ethtool to report the SQI value for
OATC14 10Base-T1S PHYs.

Open Alliance TC14 10BASE-T1S Advanced Diagnostic PHY Features
Specification ref:
https://opensig.org/wp-content/uploads/2025/06/OPEN_Alliance_10BASE-T1S_Advanced_PHY_features_for-automotive_Ethernet_V2.1b.pdf

Signed-off-by: Parthiban Veerasooran <parthiban.veerasooran@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20251201032346.6699-2-parthiban.veerasooran@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>