LICENSES: Add the MIT license

Add the full text of the MIT license to the kernel tree.  It was copied
directly from:

  https://spdx.org/licenses/MIT.html#licenseText

Add the required tags for reference and tooling.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Kate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

LICENSES/preferred/MIT

+Valid-License-Identifier: MIT
+SPDX-URL: https://spdx.org/licenses/MIT.html
+Usage-Guide:
+  To use the MIT License put the following SPDX tag/value pair into a
+  comment according to the placement guidelines in the licensing rules
+  documentation:
+    SPDX-License-Identifier: MIT
+License-Text:
+
+MIT License
+
+Copyright (c) <year> <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.