net: phy: realtek: add support for RTL8126A-integrated 5Gbps PHY

A user reported that first consumer mainboards show up with a RTL8126A
5Gbps MAC/PHY. This adds support for the integrated PHY, which is also
available stand-alone. From a PHY driver perspective it's treated the
same as the 2.5Gbps PHY's, we just have to support the new PHY ID.

Reported-by: Joe Salmeri <jmscdba@gmail.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Joe Salmeri <jmscdba@gmail.com>
Link: https://lore.kernel.org/r/0c8e67ea-6505-43d1-bd51-94e7ecd6e222@gmail.com


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