Add new PBL license

[pombredanne]

From: @0xc0170 in ARMmbed/mbed-os#13840 (comment)

@pombredanne thanks for getting back to this, we still should add the license !

The license text is https://os.mbed.com/licenses/permissive-binary-license/, the license header is:

/*
 * Copyright (c) 2019
 * SPDX-License-Identifier: LicenseRef-PBL
 *
 * This file and the related binary are licensed under the
 * Permissive Binary License, Version 1.0 (the "License");
 * you may not use these files except in compliance with the License.
 *
 * You may obtain a copy of the License here:
 * LICENSE-permissive-binary-license-1.0.txt and at
 * https://www.mbed.com/licenses/PBL-1.0
 *
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

SPDX LicenseRef-PBL. is there anything else required?

[pombredanne]

This was merged long ago.