LICENSES: Add the BSD-3-clause "Clear" license

Add the full text of the BSD 3-clause "Clear" License to the
kernel tree. It was copied directly from:

https://spdx.org/licenses/BSD-3-Clause-Clear.html

Add the required tags for reference and tooling.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Philippe Ombredanne <[e-mail]>
Reviewed-by: Kate Stewart <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

LICENSES/preferred/BSD-3-Clause-Clear

···
+Valid-License-Identifier: BSD-3-Clause-Clear
+SPDX-URL: https://spdx.org/licenses/BSD-3-Clause-Clear.html
+Usage-Guide:
+  To use the BSD 3-clause "Clear" License put the following SPDX
+  tag/value pair into a comment according to the placement guidelines in
+  the licensing rules documentation:
+    SPDX-License-Identifier: BSD-3-Clause-Clear
+License-Text:
+
+The Clear BSD License
+
+Copyright (c) [xxxx]-[xxxx] [Owner Organization]
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted (subject to the limitations in the disclaimer
+below) provided that the following conditions are met:
+
+ * Redistributions of source code must retain the above copyright notice,
+   this list of conditions and the following disclaimer.
+
+ * Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+
+ * Neither the name of [Owner Organization] nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+NO EXPRESS OR IMPLIED LICENSES TO ANY PARTY'S PATENT RIGHTS ARE GRANTED BY
+THIS LICENSE. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
+CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT
+NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER
+OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.