media: ov772x: use generic names for reset and powerdown gpios

The ov772x driver uses "rstb-gpios" and "pwdn-gpios" for reset and
powerdown pins.  However, using generic names for these gpios is
preferred.  ("reset-gpios" and "powerdown-gpios" respectively)

There is only one mainline user for these gpios, so rename to generic
names.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>