Makefile: Use common args for dtoc

At present the dtoc commmand line is repeated twice in the Makefile. Use a
variable to avoid this, so it is easier to add more conditional arguments.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index c929638..161c15b 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -307,11 +307,14 @@
 
 pythonpath = PYTHONPATH=scripts/dtc/pylibfdt
 
+DTOC_ARGS := $(pythonpath) $(srctree)/tools/dtoc/dtoc \
+	-d $(obj)/$(SPL_BIN).dtb
+
 quiet_cmd_dtocc = DTOC C  $@
-cmd_dtocc = $(pythonpath) $(srctree)/tools/dtoc/dtoc -d $(obj)/$(SPL_BIN).dtb -o $@ platdata
+cmd_dtocc = $(DTOC_ARGS) -o $@ platdata
 
 quiet_cmd_dtoch = DTOC H  $@
-cmd_dtoch = $(pythonpath) $(srctree)/tools/dtoc/dtoc -d $(obj)/$(SPL_BIN).dtb -o $@ struct
+cmd_dtoch = $(DTOC_ARGS) -o $@ struct
 
 quiet_cmd_plat = PLAT    $@
 cmd_plat = $(CC) $(c_flags) -c $< -o $(filter-out $(PHONY),$@)