arch/riscv: Add include/arch/barrier.h

mb() is used in src/arch/riscv/ and src/mainboard/emulation/*-riscv/.
It is currently provided by atomic.h, but I think it fits better into
barrier.h.

The "fence" instruction represents a full memory fence, as opposed to
variants such as "fence r, rw" which represent a partial fence. An
operating system might want to use precisely the right fence, but
coreboot doesn't need this level of performance at the cost of
simplicity.

Change-Id: I8d33ef32ad31e8fda38f6a5183210e7bd6c65815
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15830
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/arch/riscv/virtual_memory.c

@@ -14,11 +14,12 @@
  * GNU General Public License for more details.
  */
 
-#include <vm.h>
+#include <arch/barrier.h>
 #include <arch/encoding.h>
 #include <atomic.h>
-#include <stdint.h>
 #include <console/console.h>
+#include <stdint.h>
+#include <vm.h>
 
 pte_t* root_page_table;