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>