gpio: tegra186: Derive register offsets from bank/port

The register offsets for a given bank and port can be easily derived
from the bank and port indices. Update the port descriptors to list only
the bank and port numbers to simplify this.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>