media: videodev.h: add missing p_hdr10_* pointers

The HDR10 standard compound controls were missing the corresponding
pointers in videodev2.h. Add these and document them.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>