dm: sandbox: mmc: Enable building MMC code for sandbox

Enable building the MMC code for sandbox. This increases build
coverage for sandbox.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index c51d4cd..23a0c40 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -215,4 +215,6 @@
 #define CONFIG_SYS_SYSTEMACE_WIDTH	16
 #define CONFIG_SYS_SYSTEMACE_BASE	0
 
+#define CONFIG_GENERIC_MMC
+
 #endif