fdt: Allow indicating a node is for U-Boot proper only

At present it is not possible to specify that a node should be used before
relocation (in U-Boot proper) without it also ending up in SPL and TPL
device trees. Add a new "u-boot,dm-pre-proper" boolean property for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/ofnode.c b/drivers/core/ofnode.c
index fbc6751..b7b7ad3 100644
--- a/drivers/core/ofnode.c
+++ b/drivers/core/ofnode.c
@@ -699,6 +699,8 @@
 {
 	if (ofnode_read_bool(node, "u-boot,dm-pre-reloc"))
 		return true;
+	if (ofnode_read_bool(node, "u-boot,dm-pre-proper"))
+		return true;
 
 #ifdef CONFIG_TPL_BUILD
 	if (ofnode_read_bool(node, "u-boot,dm-tpl"))