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 <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Baoquan He <[e-mail]>
Acked-by: Eli Billauer <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/char/xillybus/Kconfig

···
 
 config XILLYBUS_OF
 	tristate "Xillybus over Device Tree"
-	depends on OF && HAS_DMA
+	depends on OF && HAS_DMA && HAS_IOMEM
 	help
 	  Set to M if you want Xillybus to find its resources from the
 	  Open Firmware Flattened Device Tree. If the target is an embedded