[S390] Remove TOPDIR from Makefile

This patch removes TOPDIR from arch/s390/kernel/Makefile.

Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>