dts/Makefile: don't use cpp -P

Recent dtc supports #line directives in the input source code, and even
uses them to generate useful line numbers in any messages it emits. Stop
passing -P to cpp, since there's no need any more.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

dts/Makefile

@@ -35,7 +35,7 @@ all:	$(obj).depend $(LIB)
 DT_BIN	:= $(obj)dt.dtb
 
 $(DT_BIN): $(TOPDIR)/board/$(VENDOR)/dts/$(DEVICE_TREE).dts
-	$(CPP) -P $(DTS_CPPFLAGS) $< -o $(DT_BIN).dts.tmp
+	$(CPP) $(DTS_CPPFLAGS) $< -o $(DT_BIN).dts.tmp
 	$(DTC) $(DTC_FLAGS) -O dtb -o ${DT_BIN} $(DT_BIN).dts.tmp
 
 process_lds = \