media: Documentation: mc: Add media_device_{init,cleanup}

embedding the :c:type:`media_device` instance in a larger driver-specific

structure.

Drivers register media device instances by calling

:c:func:`__media_device_register()` via the macro ``media_device_register()``

and unregistered by calling :c:func:`media_device_unregister()`.

Drivers initialise media device instances by calling

:c:func:`media_device_init()`. After initialising a media device instance, it is

registered by calling :c:func:`__media_device_register()` via the macro

``media_device_register()`` and unregistered by calling

:c:func:`media_device_unregister()`. An initialised media device must be

eventually cleaned up by calling :c:func:`media_device_cleanup()`.

Note that it is not allowed to unregister a media device instance that was not

previously registered, or clean up a media device instance that was not

previously initialised.

Entities

^^^^^^^^