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/scripts/Makefile.spl b/scripts/Makefile.spl
index 8ebe6a9..e83e93e 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -300,7 +300,7 @@
 	@bss_size_str=$(shell $(NM) $< | awk 'BEGIN {size = 0} /__bss_size/ {size = $$1} END {print "ibase=16; " toupper(size)}' | bc); \
 	dd if=/dev/zero of=$@ bs=1 count=$${bss_size_str} 2>/dev/null;
 
-$(obj)/$(SPL_BIN).dtb: dts/dt-$(SPL_NAME).dtb FORCE
+$(obj)/$(SPL_BIN).dtb: $(obj)/dts/dt-$(SPL_NAME).dtb FORCE
 	$(call if_changed,copy)
 
 pythonpath = PYTHONPATH=scripts/dtc/pylibfdt