PCI: tegra: Add Tegra186 PCIe support

Add Tegra186 PCIe support.  UPHY programming is performed by BPMP; PHY
enable calls are not required for Tegra186 PCIe.

Power partition ungate is done by BPMP powergate driver.  The Tegra186
DT description must include a "power-domains" property, which results in
dev->pm_domain being set.

Tested-by: Mikko Perttunen <mperttunen@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
[bhelgaas: add "power-domains" reference]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>