media: ov2740: Add support for external clock

On some ACPI platforms, such as Chromebooks the ACPI methods to
change the power-state (_PS0 and _PS3) fully take care of powering
on/off the sensor.

On other ACPI platforms, such as e.g. various ThinkPad models with
IPU6 + ov2740 sensor, the sensor driver must control the reset GPIO
and the sensor's clock itself.

Add support for having the driver control an optional clock.

Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>