net: phy: add genphy_c45_an_config_aneg

C45 configuration of 10/100 and multi-giga bit auto negotiation
advertisement is standardized. Configuration of 1000Base-T however
appears to be vendor specific. Move the generic code out of the
Marvell driver into the common phy-c45.c file.

v2:
- change function name to genphy_c45_an_config_aneg

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[hkallweit1@gmail.com: use new helper linkmode_adv_to_mii_10gbt_adv_t and split patch]
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>