PCI: tegra: Fix PCIe host power up sequence

The PCIe host power up sequence requires to program AFI(AXI to FPCI
bridge) registers first and then PCIe registers, otherwise AFI register
settings may not latch to PCIe IP.

PCIe root port starts LTSSM as soon as PCIe xrst is deasserted.
So deassert PCIe xrst after programming PCIe registers.

Modify PCIe power up sequence as follows:

- Power ungate PCIe partition
- Enable AFI clock
- Deassert AFI reset
- Program AFI registers
- Enable PCIe clock
- Deassert PCIe reset
- Program PCIe PHY
- Program PCIe pad control registers
- Program PCIe root port registers
- Deassert PCIe xrst to start LTSSM

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