Bluetooth: controller: User defined CPR intervals

Some proprietary protocols that reuse the Bluetooth LLCP logic requires
that the CPR interval has a valid range that differs from the Bluetooth
specification defined.

Vendor must implement the function (ull_vendor.h)
uint16_t ull_conn_interval_min_get(struct ll_conn *conn)
that returns the valid limit for the given connection.

Signed-off-by: Thomas Ebert Hansen <thoh@oticon.com>