xen/arm: Introduce a generic way to describe device

xen/arm: Introduce a generic way to describe device

Currently, Xen is supporting PCI and Platform device (based on Device Tree).

While Xen only supports Platform device on ARM, Xen will gain support of
PCI soon.

Some drivers, such as IOMMU drivers, may handle PCI and platform device in
the same way. Only few lines of code differs.

Rather than requesting to provide 2 set of functions (one for PCI and
one for platform device), introduce a generic structure "device" which
is embedded in each specialized device.

As x86 only supports PCI, introduce a new type device_t which will be an
alias to pci_dev for this architecture. It will avoid to add a new field
for this place.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Andrew Cooper <andrew.cooper3@citrix.com>