arch/riscv: Add missing license headers

Most of these files are original to coreboot and get the standard
coreboot GPL header.

encoding.h and atomic.h are from the riscv codebase and have their
license.

Change-Id: I32506b0ecf88be2f5794dc1e312a6cd9b2a271ad
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12906
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
diff --git a/src/arch/riscv/misc.c b/src/arch/riscv/misc.c
index 6e64028..65b8ecf 100644
--- a/src/arch/riscv/misc.c
+++ b/src/arch/riscv/misc.c
@@ -1,3 +1,16 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
 #include <delay.h>
 
 void init_timer(void)