ARM64: PCI: Support ACPI-based PCI host controller

Implement pci_acpi_scan_root() and other arch-specific calls so ARM64 can
use ACPI to setup and enumerate PCI buses.

Use memory-mapped configuration space information from either the ACPI
_CBA method or the MCFG table and the ECAM library and generic ECAM config
accessor ops.

Implement acpi_pci_bus_find_domain_nr() to retrieve the domain number from
the acpi_pci_root structure.

Implement pcibios_add_bus() and pcibios_remove_bus() to call
acpi_pci_add_bus() and acpi_pci_remove_bus() for ACPI slot management and
other configuration.

Signed-off-by: Tomasz Nowicki <tn@semihalf.com>
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>