dt-bindings: PCI: Add bindings for Brcmstb EP voltage regulators

dt-bindings: PCI: Add bindings for Brcmstb EP voltage regulators

Add bindings for Brcmstb EP voltage regulators.  A new mechanism is to be
added to the Linux PCI subsystem that will allocate and turn on/off
regulators.  These are standard regulators -- vpcie12v, vpcie3v3, and
vpcie3v3aux -- placed in the DT in the bridge node under the host bridge
device.

The use of a regulator property in the PCIe EP subnode such as
"vpcie12v-supply" depends on a pending pullreq to the pci-bus.yaml
file at

  https://github.com/devicetree-org/dt-schema/pull/63

Link: https://lore.kernel.org/r/20220106160332.2143-4-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>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>