Bluetooth: Controller: Add deinit() infrastructure

Allow the controller to be deinitialized, adding the whole chain of
calls:

- ll_deinit()
- lll_deinit()
- lll_clock_deinit()

in order to be able to turn everything off, including the controller's
refcount of the LF clock.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>