Clean up libfdt.h includes

The libfdt.h file is the definition file for libfdt.  It is unnecessary
to include other fdt header files (the necessary ones are pulled in
by libfdt.h).

Signed-off-by: Gerald Van Baren <gvb@unssw.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Stefan Roese <sr@denx.de>