net: phy: add genphy_c45_fast_retrain

Add generic fast retrain auto-negotiation function for C45 PHYs.

Signed-off-by: Luo Jie <luoj@codeaurora.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>