PCI: tegra: Refactor configuration space mapping code

Use only 4 KiB space from the available 1 GiB PCIe aperture to access
endpoint configuration space by dynamically moving the AFI_FPCI_BAR base
address. This frees more space for mapping endpoint device BARs on some
Tegra platforms.

The ->add_bus() and ->remove_bus() callbacks are now no longer needed,
so they can be removed.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
[treding@nvidia.com: various cleanups, update commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>