char: xillybus: make XILLYBUS_OF depend on HAS_IOMEM

On s390 systems (aka mainframes), it has classic channel devices for
networking and permanent storage that are currently even more common
than PCI devices. Hence it could have a fully functional s390 kernel
with CONFIG_PCI=n, then the relevant iomem mapping functions
[including ioremap(), devm_ioremap(), etc.] are not available.

Here let XILLYBUS_OF depend on HAS_IOMEM so that it won't be built
to cause below compiling error if PCI is unset:

------
ERROR: modpost: "devm_platform_ioremap_resource" [drivers/char/xillybus/xillybus_of.ko] undefined!
------

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306211329.ticOJCSv-lkp@intel.com/
Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Eli Billauer <eli.billauer@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230707135852.24292-3-bhe@redhat.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>