board revision readout

Hi,

as ED is going distribute current prototypes to general public, it would be great if they would be detectable in software to distinguish them from potentially fixed future boards. Probably best if it can be detected by u-boot, so that it can select a DT file with appropriate CPU frequencies, as it sounds like the current prototypes should be fully functional otherwise. It makes no sense to throw those boards away or "deprecate" them, I think the default OS should be able to support them indefinitely.

Gražvydas