Add copyright (as agreed with Manuel)

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/3rdparty/p256-m/p256-m/p256-m.c b/3rdparty/p256-m/p256-m/p256-m.c
index abb9ab4..53d306f 100644
--- a/3rdparty/p256-m/p256-m/p256-m.c
+++ b/3rdparty/p256-m/p256-m/p256-m.c
@@ -1,6 +1,7 @@
 /*
  * Implementation of curve P-256 (ECDH and ECDSA)
  *
+ * Copyright The Mbed TLS Contributors
  * Author: Manuel Pégourié-Gonnard.
  * SPDX-License-Identifier: Apache-2.0
  */
diff --git a/3rdparty/p256-m/p256-m/p256-m.h b/3rdparty/p256-m/p256-m/p256-m.h
index f455cf1..398c846 100644
--- a/3rdparty/p256-m/p256-m/p256-m.h
+++ b/3rdparty/p256-m/p256-m/p256-m.h
@@ -1,6 +1,7 @@
 /*
  * Interface of curve P-256 (ECDH and ECDSA)
  *
+ * Copyright The Mbed TLS Contributors
  * Author: Manuel Pégourié-Gonnard.
  * SPDX-License-Identifier: Apache-2.0
  */