Kill old, non-INTRNG code on ARM

[Warner Losh]

One last thing, even though we're in a slush, is to kill the pre-INTRNG
code.

It's required for armv6 and v7 for some time (only the Amlogic 8726 boards
remain broken there). And we're down to two families of chips which support
armv5: rt1310 and the Marvel gear. Both of them support INTRNG.

I propose we eliminate the INTRNG option and unifdef the code to remove the
undefined case.

Comments?

Warner