Some devices (SMSC9217, SMSC9218 and SMSC9221 at least) have better
timings, allowing a higher transfer speed. Create a common file
with these timings.

Performance results with iperf:

- omap-gpmc-smsc911x.dtsi => 54.9 Mbps
- omap-gpmc-smsc9221.dtsi => 92.7 Mbps
Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Tony Lindgren <tony@atomide.com>