net: phy: Add generic support for 2.5GBaseT and 5GBaseT

The 802.3bz specification, based on previous by the NBASET alliance,
defines the 2.5GBaseT and 5GBaseT link modes for ethernet traffic on
cat5e, cat6 and cat7 cables.

These mode integrate with the already defined C45 MDIO PMA/PMD registers
set that added 10G support, by defining some previously reserved bits,
and adding a new register (2.5G/5G Extended abilities).

This commit adds the required definitions in include/uapi/linux/mdio.h
to support these modes, and detect when a link-partner advertises them.

It also adds support for these mode in the generic C45 PHY
infrastructure.

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