sandbox: add a sandbox timer and basic test

Add a sandbox timer which get time from host os and a basic
test.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox.dts b/arch/sandbox/dts/sandbox.dts
index 08f72ac..720ef93 100644
--- a/arch/sandbox/dts/sandbox.dts
+++ b/arch/sandbox/dts/sandbox.dts
@@ -177,6 +177,10 @@
 		sides = <4>;
 	};
 
+	timer {
+		compatible = "sandbox,timer";
+	};
+
 	tpm {
 		compatible = "google,sandbox-tpm";
 	};