media: i2c: ov5670: Probe GPIOs

The OV5670 has a powerdown and reset pin, named respectively "PWDN" and
"XSHUTDOWN".

Optionally probe the gpios connected to the pins during the driver probe
routine.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>