Update dtc/libfdt submodule to v1.4.7

dtc v1.4.7 contains a bunch of improvements to make libfdt safer against
handling a corrupted or malicious tree, which is a good thing to have.  It
also includes an explicit fdt checking function that we'll be wanting in
future.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>