phy: add new LAN8710 and LAN8720 device ids to smsc phy driver

LAN8710 and LAN8720 are two new 10/100 ethernet PHY models. The two
share the same phy id, this patch adds it to the smsc phy driver.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>