omap3_beagle: use omap3-beagle-xm-ab.dtb for the xM AB revision

As of v3.15-rc3, omap3-beagle-xm-ab.dtb now exists with the usb hub (ehci) enabled.

For older kernels versions, cherry pick from mainline:
ef78f3869c37c480f1d58462a760a40dabc823f4

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
CC: Tom Rini <trini@ti.com>
CC: Nishanth Menon <nm@ti.com>

include/configs/omap3_beagle.h

@@ -170,7 +170,7 @@
 		"if test $beaglerev = C4; then " \
 			"setenv fdtfile omap3-beagle.dtb; fi; " \
 		"if test $beaglerev = xMAB; then " \
-			"setenv fdtfile omap3-beagle-xm.dtb; fi; " \
+			"setenv fdtfile omap3-beagle-xm-ab.dtb; fi; " \
 		"if test $beaglerev = xMC; then " \
 			"setenv fdtfile omap3-beagle-xm.dtb; fi; " \
 		"if test $fdtfile = undefined; then " \