microblaze: fix iounmap prototype

The missing 'volatile' keyword on the iounmap argument leads to lots of
harmless warnings in an allmodconfig build:

sound/pci/echoaudio/echoaudio.c:1879:10: warning: passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/include/asm/io.h

@@ -36,7 +36,7 @@ extern resource_size_t isa_mem_base;
 #ifdef CONFIG_MMU
 #define page_to_bus(page)	(page_to_phys(page))
 
-extern void iounmap(void __iomem *addr);
+extern void iounmap(volatile void __iomem *addr);
 
 extern void __iomem *ioremap(phys_addr_t address, unsigned long size);
 #define ioremap_nocache(addr, size)		ioremap((addr), (size))

arch/microblaze/mm/pgtable.c

@@ -127,7 +127,7 @@ void __iomem *ioremap(phys_addr_t addr, unsigned long size)
 }
 EXPORT_SYMBOL(ioremap);
 
-void iounmap(void __iomem *addr)
+void iounmap(volatile void __iomem *addr)
 {
 	if ((__force void *)addr > high_memory &&
 					(unsigned long) addr < ioremap_bot)