ARM64 / ACPI: Introduce PCI stub functions for ACPI

CONFIG_ACPI depends CONFIG_PCI on x86 and ia64, in ARM64 server
world we will have PCIe in most cases, but some of them may not,
make CONFIG_ACPI depend CONFIG_PCI on ARM64 will satisfy both.

With that case, we need some arch dependent PCI functions to
access the config space before the PCI root bridge is created, and
pci_acpi_scan_root() to create the PCI root bus. So introduce
some stub function here to make ACPI core compile and revisit
them later when implemented on ARM64.

CC: Liviu Dudau <Liviu.Dudau@arm.com>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <will.deacon@arm.com>
Tested-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Tested-by: Yijing Wang <wangyijing@huawei.com>
Tested-by: Mark Langsdorf <mlangsdo@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>
Tested-by: Timur Tabi <timur@codeaurora.org>
Tested-by: Robert Richter <rrichter@cavium.com>
Acked-by: Robert Richter <rrichter@cavium.com>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>