Restore CMakeLists.txt

Original master repository does not have CMakeLists.txt, put it back
as 'git subtree add' does not have it.

connectivity/drivers/802.15.4_RF/mcr20a-rf-driver/CMakeLists.txt

@@ -0,0 +1,14 @@
+# Copyright (c) 2020 ARM Limited. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+target_include_directories(mbed-802.15.4-rf
+    INTERFACE
+        mcr20a-rf-driver
+        source
+)
+
+target_sources(mbed-802.15.4-rf
+    INTERFACE
+        source/MCR20Drv.c
+        source/NanostackRfPhyMcr20a.cpp
+)