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/drivers/core/Makefile b/drivers/core/Makefile
index f14695b..a3fec38 100644
--- a/drivers/core/Makefile
+++ b/drivers/core/Makefile
@@ -5,5 +5,7 @@
 #
 
 obj-$(CONFIG_DM)	+= device.o lists.o root.o uclass.o util.o
+ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_OF_CONTROL) += simple-bus.o
+endif
 obj-$(CONFIG_DM_DEVICE_REMOVE)	+= device-remove.o