Revert "ARM: relax ioremap prohibition (309caa9) for -final and -stable"

This reverts commit 06c1088, as promised in the warning message.

arch/arm/mm/ioremap.c

···
 	/*
 	 * Don't allow RAM to be mapped - this causes problems with ARMv6+
 	 */
-	if (pfn_valid(pfn)) {
-		printk(KERN_WARNING "BUG: Your driver calls ioremap() on system memory.  This leads\n"
-		       KERN_WARNING "to architecturally unpredictable behaviour on ARMv6+, and ioremap()\n"
-		       KERN_WARNING "will fail in the next kernel release.  Please fix your driver.\n");
-		WARN_ON(1);
-	}
+	if (WARN_ON(pfn_valid(pfn)))
+		return NULL;
 
 	type = get_mem_type(mtype);
 	if (!type)