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>
diff --git a/board/ti/beagle/beagle.h b/board/ti/beagle/beagle.h
index 76be358..6dd2ffe 100644
--- a/board/ti/beagle/beagle.h
+++ b/board/ti/beagle/beagle.h
@@ -23,8 +23,7 @@
 #define REVISION_AXBX	0x7
 #define REVISION_CX	0x6
 #define REVISION_C4	0x5
-#define REVISION_XM_A	0x0
-#define REVISION_XM_B	0x1
+#define REVISION_XM_AB	0x0
 #define REVISION_XM_C	0x2
 
 /*