controller/iso: ISO broadcast support without encryption

Currently, the `ble_ll_iso_big.c` unconditionally uses the crypto functions. Thus the code won't compile with encryption support disabled (`BLE_LL_CFG_FEAT_LE_ENCRYPTION` unset).
For now, 23eec0a5adf44f7dd313d3e7ee882473772cc250 workaround has been merged that enables the encryption by default if `BLE_LL_ISO_BROADCASTER` is enabled.

My question is:
1. Should there be a possibility for the user to build a controller without a encryption support and Broadcast ISO support, so that the user would be able to send and/or receive unencrypted broadcast only, or
2. Should there be an restriction for `BLE_LL_ISO_BROADCASTER` with `BLE_LL_CFG_FEAT_LE_ENCRYPTION` dependency