dm: Makefile: Build of-platdata before SPL

Since SPL needs the of-platdata structures, build these before starting
to build any SPL components.

Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1318,7 +1318,8 @@ u-boot.lds: $(LDSCRIPT) prepare FORCE
 
 spl/u-boot-spl.bin: spl/u-boot-spl
 	@:
-spl/u-boot-spl: tools prepare $(if $(CONFIG_OF_SEPARATE),dts/dt.dtb)
+spl/u-boot-spl: tools prepare \
+		$(if $(CONFIG_OF_SEPARATE)$(CONFIG_SPL_OF_PLATDATA),dts/dt.dtb)
 	$(Q)$(MAKE) obj=spl -f $(srctree)/scripts/Makefile.spl all
 
 spl/sunxi-spl.bin: spl/u-boot-spl