dtoc: Fix properties with a single zero-arg phandle

At present a property with a single phandle looks like an integer value
to dtoc. Correct this by adjusting it in the phandle-processing code.

Add a test for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/dtoc_test_phandle.dts b/tools/dtoc/dtoc_test_phandle.dts
index 91dfec5..a71acff 100644
--- a/tools/dtoc/dtoc_test_phandle.dts
+++ b/tools/dtoc/dtoc_test_phandle.dts
@@ -33,4 +33,10 @@
 		compatible = "source";
 		clocks = <&phandle &phandle_1 11 &phandle_2 12 13 &phandle>;
 	};
+
+	phandle-source2 {
+		u-boot,dm-pre-reloc;
+		compatible = "source";
+		clocks = <&phandle>;
+	};
 };