Update copyright notices to use Linux Foundation guidance

As a result, the copyright of contributors other than Arm is now
acknowledged, and the years of publishing are no longer tracked in the
source files.

Also remove the now-redundant lines declaring that the files are part of
MbedTLS.

This commit was generated using the following script:

# ========================
#!/bin/sh

# Find files
find '(' -path './.git' -o -path './3rdparty' ')' -prune -o -type f -print | xargs sed -bi '

# Replace copyright attribution line
s/Copyright.*Arm.*/Copyright The Mbed TLS Contributors/I

# Remove redundant declaration and the preceding line
$!N
/This file is part of Mbed TLS/Id
P
D
'
# ========================

Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>

library/platform.c

@@ -1,7 +1,7 @@
 /*
  *  Platform abstraction layer
  *
- *  Copyright (C) 2006-2016, ARM Limited, All Rights Reserved
+ *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
  *
  *  This file is provided under the Apache License 2.0, or the
@@ -42,8 +42,6 @@
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *
  *  **********
- *
- *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
 #if !defined(MBEDTLS_CONFIG_FILE)