ramoops: add HAS_IOMEM dependency

The driver fails to compile on s390:

drivers/char/ramoops.c: In function 'ramoops_init':
drivers/char/ramoops.c:122: error: implicit declaration of function 'ioremap'

Since we won't make use of the driver anyway on s390 just let it depend on
HAS_IOMEM.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Marco Stornelli <marco.stornelli@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>