dm: test: usb: sandbox: Add keyboard tests for sandbox

Add a test that verifies that USB keyboards work correctly on sandbox.
This verifies some additional parts of the USB stack.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 52749c0..b6d9a15 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -325,6 +325,11 @@
 					sandbox,filepath = "testflash2.bin";
 				};
 
+				keyb@3 {
+					reg = <3>;
+					compatible = "sandbox,usb-keyb";
+				};
+
 			};
 		};
 	};