powerpc: clean-up include ordering in prom.h

Now that the core OF headers don't depend on prom.h, rearrange the
includes. There are still lots of implicit includes in the powerpc tree,
so the includes of OF headers are still necessary.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@linaro.org>