media: atomisp: Add support for sensors with a separate ISP v4l2_subdev

Some sensors have an ISP inside the sensor which the sensor driver models
as a separate v4l2-subdev, like the mt9m114 sensor-driver does.

Since the atomisp driver emulates a standard /dev/video# v4l2-device
without requiring the application to be aware of media-controller centric
/dev/video# devices this requires some special handling in the driver.

Add support for this setup to the atomisp driver.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>