README: Remove witherspoon-128 and reorder it

The witherspoon-128 machine was removed with openbmc/openbmc commit
40c2946. Remove it from the README and re-arrange the machine names
alphabetically so it's easier to find and maintain the names.

Change-Id: I02a88bc85167b42622c2c3db8e4e3b43db4444e2
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>