xen/arm: pci: Add stubs to allow selecting HAS_PCI

xen/arm: pci: Add stubs to allow selecting HAS_PCI

In a follow-up we will enable PCI support in Xen on Arm (i.e select
HAS_PCI).

The generic code expects the arch to implement a few functions:
arch_iommu_use_permitted()
arch_pci_clean_pirqs()

Note that this is not yet sufficient to enable HAS_PCI and will be
addressed in follow-ups.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>