media: Documentation: staging/ipu3-imgu: Add license information

The driver documentation is under GPL v2 and the uAPI documentation under
GNU FDL 1.1+ (without invariant sections) or GPL v2.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>