s390/mm: do not include <asm-generic/io.h> directly

s390/mm: do not include <asm-generic/io.h> directly

We should always include <asm/io.h> in ARCH, but not <asm-generic/io.h>
directly. Otherwise, macro defined by ARCH won't be seen and could cause
building error.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306100105.8GHnoMCP-lkp@intel.com/
Link: https://lore.kernel.org/all/ZIWrtFMUnRfVP5h0@MiWiFi-R3L-srv/
Signed-off-by: Baoquan He <bhe@redhat.com>
[agordeev@linux.ibm.com changed patch description]
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>