fdt: ARM: Add device tree control of U-Boot (CONFIG_OF_CONTROL)

This adds a device tree pointer to the global data. It can be set by
board code. A later commit will add support for making a device
tree binary blob available to U-Boot for run-time configuration.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 2dde066..d618957 100644
--- a/README
+++ b/README
@@ -816,6 +816,17 @@
 
 		XXX - this list needs to get updated!
 
+- Device tree:
+		CONFIG_OF_CONTROL
+		If this variable is defined, U-Boot will use a device tree
+		to configure its devices, instead of relying on statically
+		compiled #defines in the board file. This option is
+		experimental and only available on a few boards. The device
+		tree is available in the global data as gd->fdt_blob.
+
+		U-Boot needs to get its device tree from somewhere. This will
+		be enabled in a future patch.
+
 - Watchdog:
 		CONFIG_WATCHDOG
 		If this variable is defined, it enables watchdog