Bluetooth: controller: Add BT_CTLR_CONN_RSSI_EVENT option

Split BT_CTLR_CONN_RSSI option into two, the base option enables the
Read RSSI command, while the new BT_CTLR_CONN_RSSI_EVENT enables the
connection RSSI events. There is no handling of RSSI events, only a
BT_INFO log.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>