dm: sandbox: cros_ec: Move sandbox cros_ec to driver module

Adjust the sandbox cros_ec emulation driver to work with driver model, and
switch over to driver model for sandbox cros_ec.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 46f67be..ee4b244 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -32,6 +32,7 @@
 #define CONFIG_DM_GPIO
 #define CONFIG_DM_TEST
 #define CONFIG_DM_SERIAL
+#define CONFIG_DM_CROS_EC
 
 #define CONFIG_SYS_STDIO_DEREGISTER