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>

Makefile

@@ -1524,7 +1524,7 @@ $(defaultenv_h): $(CONFIG_DEFAULT_ENV_FILE:"%"=%) FORCE
 # ---------------------------------------------------------------------------
 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)

scripts/Makefile.spl

@@ -371,7 +371,7 @@ $(u-boot-spl-dirs): $(u-boot-spl-platdata)
 
 quiet_cmd_cpp_lds = LDS     $@
 cmd_cpp_lds = $(CPP) -Wp,-MD,$(depfile) $(cpp_flags) $(LDPPFLAGS) -ansi \
-		-D__ASSEMBLY__ -x assembler-with-cpp -P -o $@ $<
+		-D__ASSEMBLY__ -x assembler-with-cpp -std=c99 -P -o $@ $<
 
 $(obj)/u-boot-spl.lds: $(LDSCRIPT) FORCE
 	$(call if_changed_dep,cpp_lds)