powerpc: Update in-kernel dtc and libfdt to version 1.2.0

Some time ago, a copies of the upstream dtc and libfdt sources were
included in the kernel tree to avoid having these as external
dependencies for building the kernel.  Since then development on the
upstream dtc and libfdt has continued.  This updates the in-kernel
versions to match the recently released upstream dtc version 1.2.0.
This includes a number of bugfixes, many cleanups and a few new
features.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>