PCI: shpchp: Request SHPC control via _OSC when adding host bridge

The SHPC driver now must be builtin (it cannot be a module).  If it is
present, request SHPC control immediately when adding the ACPI host bridge.
This is similar to how we handle native PCIe hotplug via pciehp.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
[bhelgaas: split to separate patch]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>