media: ov5647: Add basic support for multiple sensor modes.

Specifically:

Added a structure ov5647_mode and a list of supported_modes (though no
actual new modes as yet). The state object points to the "current mode".

ov5647_enum_mbus_code, ov5647_enum_frame_size, ov5647_set_fmt and
ov5647_get_fmt all needed upgrading to cope with multiple modes.

__sensor_init (which writes all the registers) is now called by
ov5647_stream_on (once the mode is known) rather than by
ov5647_sensor_power.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Signed-off-by: Naushir Patuck <naush@raspberrypi.com>