media: mc: Add nop implementations of media_device_{init,cleanup}

To support compilation with Media controller disabled, drivers were
required to conditionally call media_device_init and media_device_cleanup.
Add nop implementations of both so drivers don't need to care (or at least
care less).

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>