Remove SPL undefine of CONFIG_OF_CONTROL

Allow SPL to be built with this option so that we can support device tree
control. Disable the simple bus for now in SPL. It may be needed later.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.uncmd_spl b/scripts/Makefile.uncmd_spl
index 343c3fc..4f05652 100644
--- a/scripts/Makefile.uncmd_spl
+++ b/scripts/Makefile.uncmd_spl
@@ -3,7 +3,9 @@
 # TODO: Invent a better way
 
 ifdef CONFIG_SPL_BUILD
+ifdef CONFIG_SPL_DISABLE_OF_CONTROL
 CONFIG_OF_CONTROL=
+endif
 
 ifndef CONFIG_SPL_DM
 CONFIG_DM_SERIAL=