dm: core: Add ofnode functions to obtain an oftree

At present dm_test_ofnode_root() does this manually. Add some inline
functions to handle it, so this code can be centralised.

Add oftree functions to produce a null tree and to check whether a tree
is valid or not.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/ofnode.c b/test/dm/ofnode.c
index eac0c50..f146b52 100644
--- a/test/dm/ofnode.c
+++ b/test/dm/ofnode.c
@@ -518,13 +518,16 @@
 	ut_assert(ofnode_valid(node));
 	ut_asserteq_str("sbe5", ofnode_get_name(node));
 
+	ut_assert(!oftree_valid(oftree_null()));
+
 	ut_assertok(make_ofnode_fdt(uts, fdt, sizeof(fdt)));
 	if (of_live_active()) {
 		ut_assertok(unflatten_device_tree(fdt, &root));
-		tree.np = root;
+		tree = oftree_from_np(root);
 	} else {
-		tree.fdt = fdt;
+		tree = oftree_from_fdt(fdt);
 	}
+	ut_assert(oftree_valid(tree));
 
 	/* Make sure they don't work on this new tree */
 	node = ofnode_path_root(tree, "mmc0");