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>
8 files changed