In the dtc tree, both flat_dt.h and libfdt/fdt.h have structures and
constants relating to the flattened device tree format derived from
asm-powerpc/prom.h in the kernel. The former is used in dtc, the
latter in libfdt.
libfdt/fdt.h is the more recent, revised version, so use that
throughout, removing flat_dt.h.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>