xen/arm: Add new device type for PCI

xen/arm: Add new device type for PCI

Add new device type (DEV_PCI) to distinguish PCI devices from platform
DT devices, so some drivers, like IOMMU, can handle PCI devices
differently.

Also add a helper which is when given a struct device returns the
corresponding struct pci_dev which this device is a part of.

Because of the header cross-dependencies, e.g. we need both
struct pci_dev and struct arch_pci_dev at the same time, this cannot be
done with an inline.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Tested-by: Rahul Singh <rahul.singh@arm.com>