fdt: Add an option to disable device tree in SPL

Some boards cannot support device tree due to lack of memory. Add an option
to allow these boards to continue to work (and even use driver model).
This is a 'negative' option since most boards are expected to support device
tree in SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>

dts/Kconfig

@@ -1,9 +1,6 @@
 #
 # Device Tree Control
 #
-# TODO:
-#   This feature is not currently supported for SPL,
-#    but this restriction should be removed in the future.
 
 config SUPPORT_OF_CONTROL
 	bool
@@ -17,6 +14,14 @@ config OF_CONTROL
 	  This feature provides for run-time configuration of U-Boot
 	  via a flattened device tree.
 
+config SPL_DISABLE_OF_CONTROL
+	bool "Disable run-time configuration via Device Tree in SPL"
+	depends on OF_CONTROL
+	help
+	  Some boards use device tree in U-Boot but only have 4KB of SRAM
+	  which is not enough to support device tree. Enable this option to
+	  allow such boards to be supported by U-Boot SPL.
+
 choice
 	prompt "Provider of DTB for DT control"
 	depends on OF_CONTROL