xen/arm: Mark device as PCI while creating one

xen/arm: Mark device as PCI while creating one

While adding a PCI device mark it as such, so other frameworks
can distinguish it from DT devices.
For that introduce an architecture defined helper which may perform
additional initialization of the newly created PCI device.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
[applicable parts]
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Tested-by: Rahul Singh <rahul.singh@arm.com>