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>