net: phy: Add helpers to determine if PHY driver is generic

We are already checking in phy_detach() that the PHY driver is of
generic kind (1G or 10G) and we are going to make use of that in the SFP
layer as well for 1000BaseT SFP modules, so expose helper functions to
return that information.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>