phy: Centralise print about attached phy

Many Ethernet drivers contain the same netdev_info() print statement
about the attached phy. Move it into the phy device code. Additionally
add a varargs function which can be used to append additional
information.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>