dts: Disable device tree for SPL on all boards

We plan to enable device tree in SPL by default. Before doing this,
explicitly disable it for all boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/zynq_zybo_defconfig b/configs/zynq_zybo_defconfig
index f119532..1849c74 100644
--- a/configs/zynq_zybo_defconfig
+++ b/configs/zynq_zybo_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_ZYNQ=y
 CONFIG_TARGET_ZYNQ_ZYBO=y
+CONFIG_SPL_DISABLE_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="zynq-zybo"
 # CONFIG_SYS_MALLOC_F is not set
 CONFIG_SPL=y