[media] DocBook: Fix documentation for struct v4l2_dv_timings

Fix this warning:
	include/media/v4l2-dv-timings.h:29: warning: cannot understand function prototype: 'const struct v4l2_dv_timings v4l2_dv_timings_presets[]; '

Unfortunately, currently, it seems that doc-nano doesn't support
documenting extern static vars.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>