media: i2c: ov5675: add .get_selection support

The sensor has 2592*1944 active pixels, surrounded by 16 active dummy
pixels and there are an additional 24 black rows "at the bottom".

                     [2624]
        +-----+------------------+-----+
        |     |     16 dummy     |     |
        +-----+------------------+-----+
        |     |                  |     |
        |     |     [2592]       |     |
        |     |                  |     |
        |16   |      valid       | 16  |[2000]
        |dummy|                  |dummy|
        |     |            [1944]|     |
        |     |                  |     |
        +-----+------------------+-----+
        |     |     16 dummy     |     |
        +-----+------------------+-----+
        |     |  24 black lines  |     |
        +-----+------------------+-----+

The top-left coordinate is gotten from the registers specified in the
modes which are identical for both currently supported modes.

There are currently two modes supported by this driver: 2592*1944 and
1296*972. The second mode is obtained thanks to subsampling while
keeping the same field of view (FoV). No cropping involved, hence the
harcoded values.

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>