Bluetooth: controller: Make ULL/LLL split the default

Make the Upper and Lower Link Layer split architecture
implementation of the controller as the default when
building Zephyr Bluetooth Low Energy controller support
in applications.

Noticeable missing feature (porting) in comparison to old
architecture implementation is, Advanced scheduling of
connection events.

The missing features will subsequently be submitted
upstream.

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