Bluetooth: controller: Prepare to introduce LL split architecture

Preparation to introduce the Upper Link Layer (ULL) and
Lower Link Layer (LLL) split architecture.

- Move SoC dependent HAL to vendor specific folder.
- Preparation to split data structures into ULL and LLL
  types.
- Added more role and state conditional compilations.
- Added some work-in-progress implementation of advertising
  extensions, will be used as inspiration in the new split
  architecture work.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>