device_tree: Add overlay support

This patch adds support for merging a device tree overlay (as defined in
Documentation/dt-object-internal.txt in the dtc repository) into a base
device tree. It was adapted from depthcharge's
http://crosreview.com/1536387.

Change-Id: Ibec833cd471201bcc7a79eebf360d5f12adb8ff9
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32869
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/device_tree.h

@@ -180,6 +180,10 @@ void dt_find_bin_prop(const struct device_tree_node *node, const char *name,
 const char *dt_find_string_prop(const struct device_tree_node *node,
 				const char *name);
 
+/* Apply an overlay to a base device tree. Ownership of the overlay data passes
+   to the newly combined base tree -- do not free() or access it afterwards! */
+int dt_apply_overlay(struct device_tree *tree, struct device_tree *overlay);
+
 /*
  * Fixups to apply to a kernel's device tree before booting it.
  */