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>
diff --git a/drivers/of/Kconfig b/drivers/of/Kconfig
index cac63c9..268163d 100644
--- a/drivers/of/Kconfig
+++ b/drivers/of/Kconfig
@@ -15,6 +15,15 @@
 	  an image of the device tree that the kernel copies from Open
 	  Firmware or other boot firmware. If unsure, say Y here.
 
+config OF_SELFTEST
+	bool "Device Tree Runtime self tests"
+	help
+	  This option builds in test cases for the device tree infrastructure
+	  that are executed one at boot time, and the results dumped to the
+	  console.
+
+	  If unsure, say N here, but this option is safe to enable.
+
 config OF_FLATTREE
 	bool
 	select DTC