PCI: tegra: Enable PCIe xclk clock clamping

Enable xclk clock clamping when entering L1. Clamp threshold will
determine the time spent waiting for clock module to turn on xclk after
signaling it. Default threshold value in Tegra124 and Tegra210 is not
enough to turn on xclk clock. Increase the clamp threshold to meet the
clock module timing in Tegra124 and Tegra210. Default threshold value is
enough in Tegra20, Tegra30 and Tegra186.

Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thierry Reding <treding@nvidia.com>