media: v4l2-core: Implement v4l2_ctrl_new_std_compound

Currently compound controls do not have a simple way of initializing its
values. This results in ofuscated code with type_ops init.

This patch introduces a new field on the control with the default value
for the compound control that can be set with the brand new
v4l2_ctrl_new_std_compound function

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
[hverkuil@xs4all.nl: fix checkpatch warning]
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>