[S390] use asm-generic/cacheflush.h

The implementation of the cache flushing interfaces on the s390
is identical with the default implementation in asm-generic.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>