dm: test: Add a test for the mmc uclass

Add a test to confirm that we can probe this device. Since there is no
MMC stack support in sandbox at present, this is as far as the test goes.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index cdc2a7b..2077e85 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -180,6 +180,10 @@
 		};
 	};
 
+	mmc {
+		compatible = "sandbox,mmc";
+	};
+
 	pci: pci-controller {
 		compatible = "sandbox,pci";
 		device_type = "pci";