x86: rename arch/x86/kernel/pci-swiotlb_64.c => pci-swiotlb.c

The file is used for 32 and 64 bit since:

  commit cfb80c9e
  Author: Jeremy Fitzhardinge <jeremy@goop.org>
  Date:   Tue Dec 16 12:17:36 2008 -0800

    x86: unify pci iommu setup and allow swiotlb to compile for 32 bit

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>