Bluetooth: controller: Move LL code from driver to ll_sw

The current hci_driver.c contained all the software Link Layer
initialization code. To decouple HCI from the actual LL, most of the
functionality that is actually part of the LL has been moved to the
ll_sw folder, opening the possibility for future hardware-based LL
implementations.

Change-Id: I1b54d655568a4ec02409da2f1a0addb4d64beed0
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>