net: phy: bcm7xxx: Reduce boilerplate code for 40nm EPHY

Introduce a macro which helps adding new 40NM EPHY entries and reduces the
amount of boilerplate code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>