media: i2c: ov2685: Make reset gpio optional

In some setups XSHUTDOWN is connected to DOVDD when it's unused,
therefore treat the reset gpio as optional.

Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>