PCI: brcmstb: Add control of subdevice voltage regulators

This Broadcom STB PCIe RC driver has one port and connects directly to one
device, be it a switch or an endpoint.  We want to be able to leverage the
recently added mechanism that allocates and turns on/off subdevice
regulators.

All that needs to be done is to put the regulator DT nodes in the bridge
below host and to set the pci_ops methods add_bus and remove_bus.

Note that the pci_subdev_regulators_add_bus() method is wrapped for two
reasons:

   1. To achieve link up after the voltage regulators are turned on.

   2. If, in the case of an unsuccessful link up, to redirect any PCIe
      accesses to subdevices, e.g. the scan for DEV/ID.  This redirection
      is needed because the Broadcom PCIe HW will issue a CPU abort if such
      an access is made when the link is down.

[bhelgaas: fold in
https://lore.kernel.org/r/20220112013100.48029-1-jim2101024@gmail.com]
Link: https://lore.kernel.org/r/20220106160332.2143-7-jim2101024@gmail.com


Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>