PCI: tegra: Rearrange Tegra PCIe driver functions

Tegra PCIe has register specifications for:

 - AXI to FPCI(AFI) bridge
 - Multiple PCIe root ports
 - PCIe PHY
 - PCIe pad control

Rearrange Tegra PCIe driver functions so that each function programs
the required module only.

- tegra_pcie_enable_controller(): Program AFI module and enable PCIe
  controller
- tegra_pcie_phy_power_on(): Bring up PCIe PHY
- tegra_pcie_apply_pad_settings(): Program PCIe REFCLK pad settings
- tegra_pcie_enable_ports(): Program each root port and bring up PCIe
  link

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