acpi: Pass -D__ASSEMBLY__ when compiling ASL files

ASL files may include various U-Boot header files, but IASL compiler
does not understand any C language embedded in these header files.
To reuse those header files for ASL compiling, use __ASSEMBLY__ in
the header files to exclude everything that is not liked by IASL.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

scripts/Makefile.lib

@@ -325,7 +325,7 @@ $(obj)/%.S: $(src)/%.ttf
 # ---------------------------------------------------------------------------
 quiet_cmd_acpi_c_asl= ASL     $<
 cmd_acpi_c_asl=         \
-	$(CPP) -x assembler-with-cpp -P $(UBOOTINCLUDE) -o $<.tmp $<; \
+	$(CPP) -x assembler-with-cpp -D__ASSEMBLY__ -P $(UBOOTINCLUDE) -o $<.tmp $<; \
 	iasl -p $< -tc $<.tmp $(if $(KBUILD_VERBOSE:1=), >/dev/null); \
 	mv $(patsubst %.asl,%.hex,$<) $@