Without yet adding in the underlying code, extend the revision
checking code to return additional values for PandaBoard ES boards.
Much of the code was taken close to verbatim from U-Boot.
compile tested, and confirmed that this code behaves identically to
the original panda configure and build for the three pandas i have:

  * Rev A2 (boots)
  * Rev EA3 (boots)
  * Rev B1 ES (no output)

so while there's still work to be done, this at least represents the
necessary infrastructure to start *recognizing* panda ES boards.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Tested-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>