Bluetooth: Controller: Add chain scanning for per adv

This adds complete support for scanning for periodic advertising trains.

AUX_SYNC_IND is always scheduled from ULL as usual, then code for aux
scanning is reused to allow for AUX_CHAIN_IND scanning scheduled from
both ULL and LLL, depending on AuxPtr.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@codecoup.pl>