powerpc/boot/fdt: Add little-endian support to libfdt wrappers

For epapr-style boot, we may be little-endian. This change implements
the proper conversion for fdt*_to_cpu and cpu_to_fdt*. We also need the
full cpu_to_* and *_to_cpu macros for this.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>