treewide: fix SPDX tags on existing files

Explicitly state the license in all source files to comply with SPDX
best practices. In addition, update the copyright notice to the new
format.

Note: At the time of cherry-picking, this was applied only to
      cores/arduino/time_macros.h and
      cores/arduino/overloads.h.

Co-Authored-by: Luca Burelli <l.burelli@arduino.cc>
Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: soburi

cores/arduino/overloads.h

@@ -1,3 +1,9 @@
+/*
+ * Copyright (c) Arduino s.r.l. and/or its affiliated companies
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 #ifdef CONFIG_DAC
 
 void analogWrite(enum dacPins pinNumber, int value);