fdt: Allow C++ comments in link scripts and DT files

At present // in a device-tree file or link script causes a warning. But
this is used in the standard license header. Update the compiler flags to
use C99, which permits this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 3f2d494..aadd1ec 100644
--- a/Makefile
+++ b/Makefile
@@ -1524,7 +1524,7 @@
 # ---------------------------------------------------------------------------
 quiet_cmd_cpp_lds = LDS     $@
 cmd_cpp_lds = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) \
-		-D__ASSEMBLY__ -x assembler-with-cpp -P -o $@ $<
+		-D__ASSEMBLY__ -x assembler-with-cpp -std=c99 -P -o $@ $<
 
 u-boot.lds: $(LDSCRIPT) prepare FORCE
 	$(call if_changed_dep,cpp_lds)