beagleboard: remove RevB support for BeagleBoard Xm

As reported in http://marc.info/?l=u-boot&m=137358037827735&w=2



There is no need for the "xMB" variant, as the gpio pins used for
identification where never changed from the xMA when the newer silcon
was used for the xMB, So rename XM A revision as AB revision
and report accordingly

Reported-by: Robert Nelson <robertcnelson@gmail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>