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>