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>

arch/openrisc/include/asm/io.h

@@ -31,7 +31,7 @@
 void __iomem *ioremap(phys_addr_t offset, unsigned long size);
 
 #define iounmap iounmap
-extern void iounmap(void __iomem *addr);
+extern void iounmap(volatile void __iomem *addr);
 
 #include <asm-generic/io.h>
 

arch/openrisc/mm/ioremap.c

@@ -77,7 +77,7 @@ void __iomem *__ref ioremap(phys_addr_t addr, unsigned long size)
 }
 EXPORT_SYMBOL(ioremap);
 
-void iounmap(void __iomem *addr)
+void iounmap(volatile void __iomem *addr)
 {
 	/* If the page is from the fixmap pool then we just clear out
 	 * the fixmap mapping.