net: phy: realtek: add support for RTL8125B-internal PHY

Realtek assigned a new PHY ID for the RTL8125B-internal PHY.
It's however compatible with the RTL8125A-internal PHY.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>