In order to obtain fdt_first_subnode & fdt_next_subnode symbols from
libfdt for use by a later patch, bump the requirement for dtc to v1.4.2
& the submodule to that same version.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Yongbok Kim <yongbok.kim@imgtec.com>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>