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/microblaze-generic_defconfig b/configs/microblaze-generic_defconfig
index 5740359..1e4cf7b 100644
--- a/configs/microblaze-generic_defconfig
+++ b/configs/microblaze-generic_defconfig
@@ -4,4 +4,5 @@
 CONFIG_SPL=y
 CONFIG_CMD_NET=y
 CONFIG_OF_CONTROL=y
+CONFIG_SPL_DISABLE_OF_CONTROL=y
 CONFIG_OF_EMBED=y