The major changes here are:

(1) Combine the pmac and chrp directories into a single
`openfirmware' directory, since both use Open Firmware and the code
was very similar.

(2) Move the Open Firmware interfaces out to a `of1275' directory
and put them in separate files so we only include the ones we need.
This work is due to Leigh Brown.

(3) On PReP and embedded, get the memory size from the memory
controller.  Don't try to ask Open Firmware even on PRePs which
have it.