Add missing license file for SL libraries

Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>

nrf_802154_sl/lib/LICENSE

@@ -0,0 +1,36 @@
+Copyright (c) 2020 - 2021, Nordic Semiconductor ASA
+
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form, except as embedded into a Nordic
+   Semiconductor ASA integrated circuit in a product or a software update for
+   such product, must reproduce the above copyright notice, this list of
+   conditions and the following disclaimer in the documentation and/or other
+   materials provided with the distribution.
+
+3. Neither the name of Nordic Semiconductor ASA nor the names of its
+   contributors may be used to endorse or promote products derived from this
+   software without specific prior written permission.
+
+4. This software, with or without modification, must only be used with a
+   Nordic Semiconductor ASA integrated circuit.
+
+5. Any software provided in binary form under this license must not be reverse
+   engineered, decompiled, modified and/or disassembled.
+
+THIS SOFTWARE IS PROVIDED BY NORDIC SEMICONDUCTOR ASA "AS IS" AND ANY EXPRESS
+OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+OF MERCHANTABILITY, NONINFRINGEMENT, AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL NORDIC SEMICONDUCTOR ASA OR CONTRIBUTORS BE
+LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
+GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.