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>