dm: doc: Update device tree binding docs for new schema

Now that Linux has accepted these tags, move U-Boot over to use them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/device-tree-bindings/fsp/fsp2/apollolake/fsp-s.txt b/doc/device-tree-bindings/fsp/fsp2/apollolake/fsp-s.txt
index dc8e325..33386eb 100644
--- a/doc/device-tree-bindings/fsp/fsp2/apollolake/fsp-s.txt
+++ b/doc/device-tree-bindings/fsp/fsp2/apollolake/fsp-s.txt
@@ -474,7 +474,7 @@
 Example:
 
 &fsp_s {
-	u-boot,dm-pre-proper;
+	bootph-some-ram;
 
 	fsps,ish-enable = <0>;
 	fsps,enable-sata = <0>;