Restore CMakeLists.txt to stm-s2lp-rf-driver

Restore CMakeLists.txt to stm-s2lp-rf-driver as it is not available
in the source repository.

Author: Arto Kinnunen

connectivity/drivers/802.15.4_RF/stm-s2lp-rf-driver/CMakeLists.txt

@@ -0,0 +1,15 @@
+# Copyright (c) 2020 ARM Limited. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+target_include_directories(mbed-802.15.4-rf
+    INTERFACE
+        stm-s2lp-rf-driver
+        source
+)
+
+target_sources(mbed-802.15.4-rf
+    INTERFACE
+        source/NanostackRfPhys2lp.cpp
+        source/at24mac_s2lp.cpp
+        source/rf_configuration.c
+)