media: ddbridge: add SPDX license identifiers

Since the boilerplates and MODULE_LICENSE are now in sync regarding the
used license (GPL v2 only), add a matching GPLv2 SPDX license identifier
to all files of the ddbridge driver.

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Cc: Manfred Voelkel <mvoelkel@DigitalDevices.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>