PCI: tegra: Add support for GPIO based PERST#

Tegra PCIe has fixed per port SFIO line to signal PERST#, which can be
controlled by AFI port register. However, if a platform routes a
different GPIO to the PCIe slot, then port register cannot control it.
Add support for GPIO based PERST# signal for such platforms. GPIO number
comes from per port PCIe device tree node. PCIe driver probe doesn't
fail if per port "reset-gpios" property is not populated, so platforms
that require this workaround must make sure that the DT property is not
missed in the corresponding device tree.

Link: https://lore.kernel.org/linux-pci/20190705084850.30777-1-jonathanh@nvidia.com/


Signed-off-by: Manikanta Maddireddy <mmaddireddy@nvidia.com>
[lorenzo.pieralisi@arm.com: squashed in fix in Link]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Thierry Reding <treding@nvidia.com>