Makefile: Build SPL dtbs in the spl/ directory

Rather than putting these in the top-level dts/ directory (which is
intended for U-Boot proper), put them in the correct subdirectory for
SPL (either spl/ or tpl/). This is where other SPL targets are kept,
so this is more consistent.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/dts/Makefile b/dts/Makefile
index 4cd65ce..c0a953f 100644
--- a/dts/Makefile
+++ b/dts/Makefile
@@ -16,10 +16,11 @@
 DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
 endif
 
-$(obj)/dt-spl.dtb: $(DTB) $(objtree)/tools/fdtgrep FORCE
+spl/$(obj)/dt-spl.dtb: $(DTB) $(objtree)/tools/fdtgrep FORCE
+	mkdir -p $(dir $@)
 	$(call if_changed,fdtgrep)
 
-$(obj)/dt-tpl.dtb: $(DTB) $(objtree)/tools/fdtgrep FORCE
+tpl/$(obj)/dt-tpl.dtb: $(DTB) $(objtree)/tools/fdtgrep FORCE
 	$(call if_changed,fdtgrep)
 
 ifeq ($(CONFIG_OF_DTB_PROPS_REMOVE),y)
@@ -55,8 +56,8 @@
 endif
 
 dtbs: $(obj)/dt.dtb \
-		$(if $(CONFIG_SPL),$(obj)/dt-spl.dtb) \
-		$(if $(CONFIG_TPL),$(obj)/dt-tpl.dtb)
+		$(if $(CONFIG_SPL),spl/$(obj)/dt-spl.dtb) \
+		$(if $(CONFIG_TPL),tpl/$(obj)/dt-tpl.dtb)
 	@:
 
 clean-files := dt.dtb.S dt-spl.dtb.S dt-tpl.dtb.S