sandbox: dts: Add a serial console node

If the sandbox device tree is provided to U-Boot (with the -d flag) then it
will use the device tree version in preference to the built-in device. The
only difference is the colour.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox.dts b/arch/sandbox/dts/sandbox.dts
index efffacb..797478a 100644
--- a/arch/sandbox/dts/sandbox.dts
+++ b/arch/sandbox/dts/sandbox.dts
@@ -1,6 +1,16 @@
 /dts-v1/;
 
 / {
+	chosen {
+		stdout-path = "/serial";
+	};
+
+	/* Needs to be available prior to relocation */
+	uart0: serial {
+		compatible = "sandbox,serial";
+		sandbox,text-colour = "cyan";
+	};
+
 	triangle {
 		compatible = "demo-shape";
 		colour = "cyan";