media: Add a driver for the ov7251 camera sensor

The ov7251 sensor is a 1/7.5-Inch B&W VGA (640x480) CMOS Digital Image
Sensor from Omnivision.

The driver supports the following modes:
- 640x480 30fps
- 640x480 60fps
- 640x480 90fps

Output format is MIPI RAW 10.

The driver supports configuration via user controls for:
- exposure and gain;
- horizontal and vertical flip;
- test pattern.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>