[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;