ARM: tegra: Fix naming in GPIO DT binding header

According to the Tegra TRM, GPIOs are aggregated into /ports/ of 8 GPIOs,
not into /banks/. Fix <dt-bindings/gpio/tegra-gpio.h> to correctly reflect
this naming convention. While this seems like silly churn, it will become
slightly more important once we introduce the GPIO binding for upcoming
Tegra chips.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>