media: v4l2_ctrl: Add p_def to v4l2_ctrl_config

This allows setting the default value on compound controls created via
v4l2_ctrl_new_custom.

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