media: ov7670: Add the ov7670_s_power function

Add the ov7670_s_power function which is responsible for
manipulating the power dowm mode through the PWDN pin and the reset
operation through the RESET pin, and keep it powered at all times.

[sakari.ailus@linux.intel.com: set pwdn_gpio direction only once]
Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>