Bluetooth: controller: Add ll_adv_scan_state_cb callback

Added a callback function ll_adv_scan_state_cb from the
Controller that gets called on either an advertiser or a
scanner getting started as the Controller's first enabled
state. The callback is also called on the Controller's
last disabled advertising or scanning state.

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