PCI: hotplug: Add Ampere Altra Attention Indicator extension driver

On Ampere Altra, PCIe hotplug is handled through ACPI. A side interface is
also present to request system firmware control of the hotplug Attention
Indicators. Add an ACPI PCI Hotplug companion driver to support Attention
Indicator control.

Link: https://lore.kernel.org/r/20230930002036.6491-2-scott@os.amperecomputing.com
Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
[bhelgaas: mask domain to low 4 bits]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>