treewide: fdt: Move fdt_get_config_... to ofnode_conf_read...

The current API is outdated as it requires a devicetree pointer.

Move these functions to use the ofnode API and update this globally. Add
some tests while we are here.

Correct the call in exynos_dsim_config_parse_dt() which is obviously
wrong.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index b36447c..2dfae1c 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -62,6 +62,9 @@
 	};
 
 	config {
+		testing-bool;
+		testing-int = <123>;
+		testing-str = "testing";
 		environment {
 			from_fdt = "yes";
 			fdt_env_path = "";