[U-Boot] Removal of generated asm-offsets.s/h

Hi

If a soc automatically generates asm-offsets.s/h in its makefile it isn't removed by a make clean or make distclean. See mx35 as an example. Even adding a clean: target to the SoCs makefile doesn't help. What would be the right way to remove these files with make clean?

Regards Matthias Weißer