Makefile: Correct dependency race condition with TPL

At present we sometimes see the following build error when building on a
machine with multiple cores.

+make[2]: *** No rule to make target 'dts/dt.dtb', needed by 'tpl/u-boot-tpl.dtb'.  Stop.

Add a dependency to correct this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>

Makefile

@@ -1351,7 +1351,8 @@ spl/u-boot-spl.sfp: spl/u-boot-spl
 spl/boot.bin: spl/u-boot-spl
 	@:
 
-tpl/u-boot-tpl.bin: tools prepare
+tpl/u-boot-tpl.bin: tools prepare \
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb)
 	$(Q)$(MAKE) obj=tpl -f $(srctree)/scripts/Makefile.spl all
 
 TAG_SUBDIRS := $(patsubst %,$(srctree)/%,$(u-boot-dirs) include)