PCI: tegra: Implement accurate power supply scheme

The current description of power supplies doesn't match the hardware.
Instead it's designed to support the needs of current designs, which
will break as soon as a new design appears that cannot be described
using the current assumptions.

In order to fully support all possible future designs, all power supply
inputs to the PCIe block need to be accurately described and separately
configurable.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>