dm: memory: Introduce new uclass

Introduce UCLASS_MEMORY for future Memory Controller
device drivers.

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 70e27cd..db72c64 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -932,6 +932,10 @@
 		};
 	};
 
+	memory-controller {
+		compatible = "sandbox,memory";
+	};
+
 	misc-test {
 		#address-cells = <1>;
 		#size-cells = <1>;