et8ek8: add subdev support

Subdevices support for et8ek8: we now have subdevices connected to the
sensor, not to the isp, which makes more sense when we have ability to
switch front/back sensor.

FIXME: Subdev code should be moved to some other place -- this is
really common code.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
3 files changed