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. This mirrors an identical commit in the Linux kernel.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>