pci: tegra: introduce weak tegra_pcie_board_port_reset() function

Introduce a weak tegra_pcie_board_port_reset() function by default
calling the existing tegra_pcie_port_reset() function. Additionally add
a tegra_pcie_port_index_of_port() function to retrieve the specific PCIe
port index if required. This allows overriding the PCIe port reset
functionality from board specific code as e.g. required for Apalis T30
and Apalis TK1.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>