[MIPS] Fix iounmap argument to const volatile.

With the existing prototype the following code:

    const void __iomem *io = ioremap();
    x = readb(io);
    iounmap(io);

did result in a warning.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/mm/ioremap.c

@@ -176,7 +176,7 @@ void __iomem * __ioremap(phys_t phys_addr, phys_t size, unsigned long flags)
 
 #define IS_KSEG1(addr) (((unsigned long)(addr) & ~0x1fffffffUL) == CKSEG1)
 
-void __iounmap(volatile void __iomem *addr)
+void __iounmap(const volatile void __iomem *addr)
 {
 	struct vm_struct *p;
 

include/asm-mips/io.h

@@ -172,7 +172,7 @@ extern unsigned long isa_slot_offset;
 #define page_to_phys(page)	((dma_addr_t)page_to_pfn(page) << PAGE_SHIFT)
 
 extern void __iomem * __ioremap(phys_t offset, phys_t size, unsigned long flags);
-extern void __iounmap(volatile void __iomem *addr);
+extern void __iounmap(const volatile void __iomem *addr);
 
 static inline void __iomem * __ioremap_mode(phys_t offset, unsigned long size,
 	unsigned long flags)
@@ -279,7 +279,7 @@ static inline void __iomem * __ioremap_mode(phys_t offset, unsigned long size,
 #define ioremap_uncached_accelerated(offset, size)			\
 	__ioremap_mode((offset), (size), _CACHE_UNCACHED_ACCELERATED)
 
-static inline void iounmap(volatile void __iomem *addr)
+static inline void iounmap(const volatile void __iomem *addr)
 {
 #define __IS_KSEG1(addr) (((unsigned long)(addr) & ~0x1fffffffUL) == CKSEG1)