dt-bindings: PCI: tegra: Add sideband pins configuration entries

Add optional bindings "pinctrl-names" and "pinctrl-0" to describe pin
configuration information of a particular PCIe controller.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thierry Reding <treding@nvidia.com>