net: phy: add 802.3 clause 45 support to phylib

Add generic helpers for 802.3 clause 45 PHYs for >= 10Gbps support.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>