Bluetooth: controller: Move ll.h to an include folder

To present a common interface to both software and hardware Link Layers
the API in ll.h now lives in its own separate include/ folder.

Change-Id: I2b0ab0d11b47b9c35a5759bcc30f347e6c616648
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>