Compensate for the fact the the FDT uses a different offset for the registers than the hardcoded ones we used before.  Replace a few magic numbers with #defines while I'm there.  Based on suggestions from dms@.
Compensate for the fact the the FDT uses a different offset for the registers
than the hardcoded ones we used before.  Replace a few magic numbers with
#defines while I'm there.  Based on suggestions from dms@.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars