media: ar0521: Implement enum_frame_sizes

Implement the enum_frame_size pad operation.

The sensor supports a continuous size range of resolutions.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>