of: Add device tree selftests

Add some runtime test cases for the library of device tree parsing functions.

v2: - Add testcase for phandle with 0 args
    - Don't run testcases if testcase data isn't present in device tree

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

arch/arm/boot/dts/testcases/tests-phandle.dtsi

@@ -0,0 +1,37 @@
+
+/ {
+	testcase-data {
+		phandle-tests {
+			provider0: provider0 {
+				#phandle-cells = <0>;
+			};
+
+			provider1: provider1 {
+				#phandle-cells = <1>;
+			};
+
+			provider2: provider2 {
+				#phandle-cells = <2>;
+			};
+
+			provider3: provider3 {
+				#phandle-cells = <3>;
+			};
+
+			consumer-a {
+				phandle-list =	<&provider1 1>,
+						<&provider2 2 0>,
+						<0>,
+						<&provider3 4 4 3>,
+						<&provider2 5 100>,
+						<&provider0>,
+						<&provider1 7>;
+				phandle-list-names = "first", "second", "third";
+
+				phandle-list-bad-phandle = <12345678 0 0>;
+				phandle-list-bad-args = <&provider2 1 0>,
+							<&provider3 0>;
+			};
+		};
+	};
+};

arch/arm/boot/dts/testcases/tests.dtsi

@@ -0,0 +1 @@
+/include/ "tests-phandle.dtsi"