PCI: tegra: set up PADS_REFCLK_CFG1

The registers PADS_REFCLK_CFG are an array of 16-bit data, one entry per
PCIe root port. For Tegra30, we therefore need to write a 3rd entry in
this array. Doing so makes the mini-PCIe slot on Beaver operate correctly.

While we're at it, add some #defines to partially document the fields
within these 16-bit values.

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