staging: mt7621-dts: add missing SPDX license to files

Device tree files included here haven't got SPDX license header at all.
Hence, add it to all of them.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20211025094240.4356-1-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>