dm: test: Add a test for the ram uclass

Add a test to confirm that we can probe this device and get information on
the available RAM.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 79b0175..cdc2a7b 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -196,6 +196,10 @@
 		};
 	};
 
+	ram {
+		compatible = "sandbox,ram";
+	};
+
 	reset@0 {
 		compatible = "sandbox,warm-reset";
 	};