[U-Boot] Zoom2, Beagle board revision detection.

[Tom Rix]

This is against the arm/next branch 

Zoom2 has two meaningful board revisions : production and beta.  This change
will allow the board differences to be handled at runtime.  

The Zoom2 change is based on how the Beagle Board does its board revision
but since it is newer, it uses the OMAP3 GPIO API that was recently added. 

This was run tested on Zoom2 beta and production boards.

Since I was taking a close look at the Beagle code,  I updated it to also
use the OMAP3 GPIO API.

This was run tested on a Beagle B7 board.  A version C board was not 
available for testing.  It would be good if someone verified the Beagle change
with a versioin C board.   

Tom