powerpc: Run on old powermacs.

Old powermacs have a number of differences from current machines:
- there is no interrupt tree in the device tree, just interrupt
  or AAPL,interrupt properties
- the chosen node in the device tree is called /chosen@0
- the OF claim method doesn't map the memory, so we have to do
  an explicit map call as well
- there is no /chosen/cpu property on SMP machines
- the NVRAM isn't structured as a set of partitions.

This adapts the merged powermac support code to cope with these
issues.

Signed-off-by: Paul Mackerras <paulus@samba.org>