libfdt: fix <linux/libfdt.h>

I do not remember why, but this is apparently a file-copy mistake.
The file name is libfdt.h, but its content is that of libfdt_env.h

Re-import it from upstream Linux.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>