dt-bindings: PCI: rockchip: Add vpcie12v-supply for Rockchip PCIe controller

The PCIe connector provide a optional 12V power supply for high power
downstream components, so we add this as an optional one if we need to
control it.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/rockchip-pcie.txt

@@ -47,6 +47,7 @@ Optional Property:
 	using 24MHz OSC for RC's PHY.
 - ep-gpios: contain the entry for pre-reset gpio
 - num-lanes: number of lanes to use
+- vpcie12v-supply: The phandle to the 12v regulator to use for PCIe.
 - vpcie3v3-supply: The phandle to the 3.3v regulator to use for PCIe.
 - vpcie1v8-supply: The phandle to the 1.8v regulator to use for PCIe.
 - vpcie0v9-supply: The phandle to the 0.9v regulator to use for PCIe.