dt-bindings: PCI: tegra: Add PCIe slot supplies regulator entries

Add optional bindings "vpcie3v3-supply" and "vpcie12v-supply" to describe
regulators of a PCIe slot's supplies 3.3V and 12V provided the platform
is designed to have regulator controlled slot supplies.

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>