MFC r328641: dtc(1): Revert WARNS change from r328173

WARNS > 3 breaks xtoolchain builds to varying degrees. Revert it.

MFC r320296, r328173: Update dtc(1)

MFC r320296 (manu): dtc: Update to upstream 9ce35ff8

- Add "compatible with gpl dtc X.Y.Z" to version output so U-Boot doesn't complain
- Fix cross reference node

This fixes some Allwinner DTS (and probably others).

MFC r328173: dtc(1): Update to upstream ea3c233

Highlights of this update:
- /__local_fixups__ is now generated to be GPL dtc and libfdt compliant
- Compiling with -@ will now cause dtc to assign phandles to all labelled
nodes
- /include/ and /incbin/ now handle absolute paths correctly
- The manpage now has information about overlays, including how to apply
them and how to generate them
- Syntactic sugar for overlays is now supported, allowing an overlay DTS
like:

=
/dts-v1/;
/plugin/;

&foo {
foo,status = "okay";
};
=

to generate a fragment targetting <&foo>.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Improvements to BSD-licensed DTC.

- Added an expression parser so that expressions from headers are now working
- Fixed missing null terminators on cross references
- Disabled exceptions / RTTI in the build for smaller binaries
- Changed phandle order generation to be identical to GPL'd dtc

Ensure that dtc is built in C++11 mode.

Reported by: George Abdelmalik

Import new (BSDL) device tree compiler. Now built by default, so that it can't
be used on the host system (and not installed on the device, if required). The
GPL'd one is still available if there are any devices that need it (make
universe passes with it, including kernels that use fdt, but there may be some
out-of-tree ones). WITH_GPL_DTC can be used to select the old one, for now.

Probably won't be MFC'd, but we'll remove the GPL'd version in head after the
new one has had a lot more testing and ship it in 10.0.