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The current API is outdated as it requires a devicetree pointer.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/drivers/core/ofnode.c b/drivers/core/ofnode.c
index 701b23e..08705ef 100644
--- a/drivers/core/ofnode.c
+++ b/drivers/core/ofnode.c
@@ -1103,3 +1103,36 @@
 	else
 		return ofnode_write_string(node, "status", "disabled");
 }
+
+bool ofnode_conf_read_bool(const char *prop_name)
+{
+	ofnode node;
+
+	node = ofnode_path("/config");
+	if (!ofnode_valid(node))
+		return false;
+
+	return ofnode_read_bool(node, prop_name);
+}
+
+int ofnode_conf_read_int(const char *prop_name, int default_val)
+{
+	ofnode node;
+
+	node = ofnode_path("/config");
+	if (!ofnode_valid(node))
+		return default_val;
+
+	return ofnode_read_u32_default(node, prop_name, default_val);
+}
+
+const char *ofnode_conf_read_str(const char *prop_name)
+{
+	ofnode node;
+
+	node = ofnode_path("/config");
+	if (!ofnode_valid(node))
+		return NULL;
+
+	return ofnode_read_string(node, prop_name);
+}