sandbox: Support iotrace feature

Support the iotrace feature for sandbox, and enable it, using some dummy
I/O access methods.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/io.h b/arch/sandbox/include/asm/io.h
index 7956041..895fcb8 100644
--- a/arch/sandbox/include/asm/io.h
+++ b/arch/sandbox/include/asm/io.h
@@ -40,4 +40,14 @@
 /* Map from a pointer to our RAM buffer */
 phys_addr_t map_to_sysmem(const void *ptr);
 
+/* Define nops for sandbox I/O access */
+#define readb(addr) 0
+#define readw(addr) 0
+#define readl(addr) 0
+#define writeb(v, addr)
+#define writew(v, addr)
+#define writel(v, addr)
+
+#include <iotrace.h>
+
 #endif