Bluetooth: controller: Restructure LLL include files

Move LLL header files for all roles to common folder. Move
vendor-specific content to internal header files, in particular ADV
buffer handling. Define the vendor-specific type lll_adv_pdu in a
separate header file lll_adv_pdu.h to avoid cyclic dependency when
moving ADV buffer handling to internal header file. Make sure all
internal header files are in vendor-specific folders.

The goal is to have a more consistent structure for LLL header files
and allow unconditional inclusion of non-internal LLL header files.

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>