dm: test: Add tests for the clk uclass

Add tests of each API call using a sandbox clock device.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index c25614a..3c9abb3 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -105,6 +105,10 @@
 		compatible = "denx,u-boot-fdt-test";
 	};
 
+	clk@0 {
+		compatible = "sandbox,clk";
+	};
+
 	eth@10002000 {
 		compatible = "sandbox,eth";
 		reg = <0x10002000 0x1000>;