arm64: Fix source code file path in comments

Architecture specific code for i386 and x86_64 was unified and merged to
the arch/x86. This patch fix old path of x86 architecture in a comment
from the arch/arm64/include/asm/fixmap.h.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/fixmap.h

@@ -8,7 +8,7 @@
  * Copyright (C) 1998 Ingo Molnar
  * Copyright (C) 2013 Mark Salter <msalter@redhat.com>
  *
- * Adapted from arch/x86_64 version.
+ * Adapted from arch/x86 version.
  *
  */