Bluetooth: Controller: implemented HCI_LE_Read_ISO_TX_Sync in ISO-AL

Implemented:
-- Storing information required to service HCI_LE_Read_ISO_TX_Sync
   request
-- Implemented interface to retrieve information
-- Cleared ISO-AL source and sink state on deallocation
-- Renamed cig_ref_point to grp_ref_point as it serves a common purpose
   in both CIS and BIS

Signed-off-by: Nirosharn Amarasinghe <niag@demant.com>