net: phy: bcm7xxx: Add BCM74371 PHY ID

Add the BCM74371 PHY ID to the list of supported chips. This is a 28nm
technology Gigabit PHY SoC.

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