Add support to read the bus-type for V4L2_MBUS_BT656 and enable BT.656
mode in the sensor if needed.
For backward compatibility with older DTS where the bus-type property was
not mandatory, assume V4L2_MBUS_PARALLEL as it was the only supported bus
at the time. v4l2_fwnode_endpoint_alloc_parse() will not fail if
'bus-type' is not specified.