ARM64: PCI: Implement AML accessors for PCI_Config region

On ACPI systems, the PCI_Config OperationRegion allows AML to access PCI
configuration space.  The ACPI CA AML interpreter uses performs config
space accesses with acpi_os_read_pci_configuration() and
acpi_os_write_pci_configuration(), which are OS-dependent functions
supplied by acpi/osl.c.

Implement the arch-specific raw_pci_read() and raw_pci_write() interfaces
used by acpi/osl.c for PCI_Config accesses.

N.B. PCI_Config accesses are not supported before PCI bus enumeration.

[bhelgaas: changelog]
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>