sandbox: Add <asm/barrier.h>

Add a mostly empty asm/barrier.h file for sandbox where we define nop() to
be an empty function.

Reviewed-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/sandbox/include/asm/barrier.h b/arch/sandbox/include/asm/barrier.h
new file mode 100644
index 0000000..0928a78
--- /dev/null
+++ b/arch/sandbox/include/asm/barrier.h
@@ -0,0 +1,3 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#define nop()