xen/iommu: smmu: Add Xen specific code to be able to use the driver

xen/iommu: smmu: Add Xen specific code to be able to use the driver

The main goal is to modify as little the Linux code to be able to port
easily new feature added in Linux repo for the driver.

To achieve that we:
    - Add helpers to Linux function not implemented on Xen
    - Add callbacks used by Xen to do our own stuff and call Linux ones
    - Only modify when required the code which comes from Linux. If so a
    comment has been added with /* Xen: ... */ explaining why it's
    necessary.

The support for PCI has been commented because it's not yet supported by
Xen ARM and therefore won't compile.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fix a couple of typos in comments ]