Improve r290373, do a runtime check rather than a compile time switch. I
learned that the Power8 and the PS3 have a mix of OFW and FDT. Both have AIM
defined. But currently they are not affected. They have no I2C devices under
OFW.

This version was tested on a Quad G5 and build tested for armv6*.

Discussed with nwhitehorn@
Reviewed by: ian@