fdt: Add fdt_translate_address to convert reg node to cpu phys addr

This code is extracted out of the Linux Kernel code from
arch/powerpc/kernel/prom_parse.c.

We maintain some of the same structure to support multiple bus types even
though we only have one in the current code.  In the future we might want
to translate across a PCI bus and thus it will be easier to add that
functionality back in.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>