Update dtc submodule to v1.4.3

Since the last submodule update (which was v1.4.2) dtc and libfdt have
gained some features which would be useful in qemu.  There's now a v1.4.3
upstream release, so update our submodule to point to it.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

dtc @ fa8bc7f9

-Subproject commit ec02b34c05be04f249ffaaca4b666f5246877dea
+Subproject commit fa8bc7f928ac25f23532afc8beb2073efc8fb063