PCI: tegra: Add loadable kernel module support

Implement remove callback function for Tegra PCIe driver to add
loadable kernel module support.

Per PCIe r3.0, sec 5.3.3.2.1, PCIe root port should broadcast PME_Turn_Off
message before PCIe link goes to L2. PME_Turn_Off broadcast mechanism is
implemented in AFI module. Each Tegra PCIe root port has its own
PME_Turn_Off and PME_TO_Ack bitmap in AFI_PME register, program this
register to broadcast PME_Turn_Off message.

Once PME_TO_Ack is recieved driver will turn OFF PCIe clock, power gate
PCIe partition and turn OFF regulators.

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