net: phy: aquantia: inform about proprietary 1000Base-T2 mode being in use

The AQCS109 supports a proprietary 2-pair 1Gbps mode. The standard
registers don't allow to tell between 1000BaseT and 1000BaseT2.
Add reporting this proprietary mode based on a vendor register.

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