Add .editorconfig file

Author: ladislas

.editorconfig

@@ -0,0 +1,21 @@
+# Copyright (c) 2021 ARM Limited. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+# Mbed OS Coding style spec for editors.
+
+# References:
+# [EC]          http://editorconfig.org/
+
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.yml]
+indent_style = space
+indent_size = 2