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