iommu: Move swap_pci_ref function to drivers/iommu/pci.h.

The swap_pci_ref function is used by the IOMMU API code for
swapping pci device pointers, while determining the iommu
group for the device.
Currently this function was being implemented for different
IOMMU drivers.  This patch moves the function to a new file,
drivers/iommu/pci.h so that the implementation can be
shared across various IOMMU drivers.

Signed-off-by: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>