dtc: Sync with upstream commit 26a0fe5

- 0206c0f ("Handle top-level /delete-node/ directives.")
- d612a9e ("Remove C++11 standard constrain")
- Remove extra white lines after the $FreeBSD$ tag removal

Reviewed by: kevans (earlier), theraven, emaste
Differential Revision: https://reviews.freebsd.org/D41482

Remove $FreeBSD$: one-line bare tag

Remove /^\s*\$FreeBSD\$$\n/

dtc(1): Update to 0892ec7; HACKING and implicit header fixes

Fixes courtesy of arichardson and jmg:
- HACKING was pointing to the wrong place
- Added headers were being relied on implicitly, but libstdc++ did not
comply with the unspoken wishes of dtc.

MFC after: 1 week

Update some obsolete information in the HACKING document.

Reported by: bapt

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.