sandbox: add a sandbox timer and basic test

Add a sandbox timer which get time from host os and a basic
test.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 2a68203..b0fe501 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -19,7 +19,9 @@
 #define CONFIG_IO_TRACE
 #define CONFIG_CMD_IOTRACE
 
+#ifndef CONFIG_TIMER
 #define CONFIG_SYS_TIMER_RATE		1000000
+#endif
 
 #define CONFIG_SYS_STDIO_DEREGISTER