V4L/DVB (13618): v4l2: Adding helper function to get dv preset description

This patch adds a helper function to get description of a digital
video preset added by the video timing API. This will be useful for drivers
implementing the above API.

Signed-off-by: Muralidharan Karicheri <m-karicheri2@ti.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>