dt-bindings: pci: tegra: Document PCIe DPD pinctrl optional prop

Document PCIe DPD pinctrl optional property to put PEX clk & BIAS pads
in low power mode.

Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/pci/nvidia,tegra20-pcie.txt

@@ -65,6 +65,14 @@ Required properties:
   - afi
   - pcie_x
 
+Optional properties:
+- pinctrl-names: A list of pinctrl state names. Must contain the following
+  entries:
+  - "default": active state, puts PCIe I/O out of deep power down state
+  - "idle": puts PCIe I/O into deep power down state
+- pinctrl-0: phandle for the default/active state of pin configurations.
+- pinctrl-1: phandle for the idle state of pin configurations.
+
 Required properties on Tegra124 and later (deprecated):
 - phys: Must contain an entry for each entry in phy-names.
 - phy-names: Must include the following entries: