ARM: Add SMSC911X support to Overo platform (V2)

Gumstix will soon be shipping a variant of their Summit board that
includes an SMSC LAN9221 ethernet interface.  This patch provides
support via the smsc911x driver when enabled in kernel config.

The Overo defconfig is not updated since the LAN9221 is an option
not present on all systems.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Steve Glendinning <steve.glendinning@smsc.com>