xen/arm: make pci_host_common_probe return the bridge

Some of the PCI host bridges require additional processing during the
probe phase. For that they need to access struct bridge of the probed
host, so return pointer to the new bridge from pci_host_common_probe.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>