Bluetooth: Controller: Add abort fairness in overlapping Periodic Sync

Add implementation in Periodic Sync LLL abort to use near
fair scheduling to ensure overlapping multiple Periodic
Sync set each get to use the radio and not lead to Sync
Loss.

The implementation is used when overlap in radio event
happens in unreserved time space of the event. An abort
count is maintain in each Periodic Sync instance and is
used to decide whether the current event or the next
event is to be aborted when they overlap.

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