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: David Gibson <david@gibson.dropbear.id.au>