openrisc: io: Define iounmap argument as volatile

When OpenRISC enables PCI it allows for more drivers to be compiled
resulting in exposing the following with -Werror.

    drivers/video/fbdev/riva/fbdev.c: In function 'rivafb_probe':
    drivers/video/fbdev/riva/fbdev.c:2062:42: error:
	    passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type

    drivers/video/fbdev/nvidia/nvidia.c: In function 'nvidiafb_probe':
    drivers/video/fbdev/nvidia/nvidia.c:1414:20: error:
	    passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type

    drivers/scsi/aic7xxx/aic7xxx_osm.c: In function 'ahc_platform_free':
    drivers/scsi/aic7xxx/aic7xxx_osm.c:1231:41: error:
	    passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type

Most architectures define the iounmap argument to be volatile.  To fix this
issue we do the same for OpenRISC.  This patch must go before PCI is enabled on
OpenRISC to avoid any compile failures.

Link: https://lore.kernel.org/lkml/20220729033728.GA2195022@roeck-us.net/
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Stafford Horne <shorne@gmail.com>