PCI: Move domain assignment from arm64 to generic code

The current logic in arm64 pci_bus_assign_domain_nr() is flawed in that
depending on the host controller configuration for a platform and the
initialization sequence, core code may end up allocating PCI domain numbers
from both DT and the generic domain counter, which would result in PCI
domain allocation aliases/errors.

Fix the logic behind the PCI domain number assignment and move the
resulting code to the PCI core so the same domain allocation logic is used
on all platforms that select CONFIG_PCI_DOMAINS_GENERIC.

[bhelgaas: tidy changelog]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
CC: Rob Herring <robh+dt@kernel.org>
CC: Catalin Marinas <catalin.marinas@arm.com>