xen/arm: correct comment in enlighten.c

Correct a comment in arch/arm/xen/enlighten.c referencing a wrong
source file.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>