Bluetooth: controller: add functions to enable CTE req LLCP

Add functions that handle enable CTE request control procedure
request from host.
Implementation of ll_df_set_conn_cte_req_enable in ULL is partial.
It will be integrated with refactored implementation of control
procedures.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>