spdx: add 2-BSD SPDX tags on relicensed files

Updates the recently merged relicensed files with the 2-Clause BSD
SPDX annotation.

Closes #5208.

bsps/shared/dev/clock/bcm2835-system-timer.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+
 /**
  * @file
  *