net: phy: realtek: add support for the 2.5Gbps PHY in RTL8125

This adds support for the integrated 2.5Gbps PHY in Realtek RTL8125.
Advertisement of 2.5Gbps mode is done via a vendor-specific register.
Same applies to reading NBase-T link partner advertisement.
Unfortunately this 2.5Gbps PHY shares the PHY ID with the integrated
1Gbps PHY's in other Realtek network chips and so far no method is
known to differentiate them. As a workaround use a dedicated fake PHY ID
that is set by the network driver by intercepting the MDIO PHY ID read.

v2:
- Create dedicated PHY driver and use a fake PHY ID that is injected by
  the network driver. Suggested by Andrew Lunn.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>