Provide support for enabling and disabling regulator supplies to control
power to the camera sensor.

While updating the power on function, document that a sleep is
represented as 'T4' in the datasheet power on sequence.
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>