fdt: Adjust DEFAULT_DEVICE_TREE to device on OF_CONTROL

This option has no meaning without OF_CONTROL, so add a dependency.

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

dts/Kconfig

@@ -53,6 +53,7 @@ endchoice
 
 config DEFAULT_DEVICE_TREE
 	string "Default Device Tree for DT control"
+	depends on OF_CONTROL
 	help
 	  This option specifies the default Device Tree used for DT control.
 	  It can be overridden from the command line: