libfdt: change libfdt_internal.h to a wrapper of scripts/dtc/libfdt/*

Fortunately, U-Boot did not modify libfdt_internal.h locally.

Change it to a wrapper of scripts/dtc/libfdt/fdt.h, which will be
periodically synced with the upstream DTC (or kernel).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>