virtio: add HAS_IOMEM dependency to MMIO platform bus driver

Fix this compile error on s390:

  CC [M]  drivers/virtio/virtio_mmio.o
drivers/virtio/virtio_mmio.c: In function 'vm_get_features':
drivers/virtio/virtio_mmio.c:107:2: error: implicit declaration of function 'writel'

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/Kconfig

@@ -37,7 +37,7 @@ config VIRTIO_BALLOON
 
  config VIRTIO_MMIO
  	tristate "Platform bus driver for memory mapped virtio devices (EXPERIMENTAL)"
- 	depends on EXPERIMENTAL
+ 	depends on HAS_IOMEM && EXPERIMENTAL
  	select VIRTIO
  	select VIRTIO_RING
  	---help---