smc911x: Add support for LAN921{5,7,8} chips from SMSC

LAN92{5,7,8} chips from SMSC are register compatible with LAN911{5,6,7,8}
controllers, and only add support for HP Auto-MDIX. LAN9218 doesn't have
an external MII interface.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>