[Silabs] SiWx917 folder structure skeleton in silabs folder

examples/lighting-app/silabs/SiWx917/.gn

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+# Copyright (c) 2020 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/build.gni")
+
+# The location of the build configuration file.
+buildconfig = "${build_root}/config/BUILDCONFIG.gn"
+
+# CHIP uses angle bracket includes.
+check_system_includes = true
+
+default_args = {
+  target_cpu = "arm"
+  target_os = "freertos"
+  chip_openthread_ftd = true
+
+  import("//args.gni")
+}