FDT: don't use private kernel header files

On some systems (for example Fedora Core 4) U-Boot builds with the
following wanrings only:

...
In file included from /home/wd/git/u-boot/include/libfdt_env.h:33,
                 from fdt.c:51:
		 /usr/include/asm/byteorder.h:6:2: warning: #warning using private kernel header; include <endian.h> instead!

This patch fixes this problem.

Signed-off-by: Wolfgang Denk <wd@denx.de>