ata: ahci_xgene: use correct type for port mmio address

Sparse complains about an incorrect type for port_mmio pointer
variables:

drivers/ata/ahci_xgene.c:196:41: warning: incorrect type in initializer
(different address spaces)
drivers/ata/ahci_xgene.c:196:41:    expected void *port_mmio
drivers/ata/ahci_xgene.c:196:41:    got void [noderef] __iomem *

Fix this by declaring port_mmio as "void __iomem *" instead of "void *".

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>