ARM: tegra: convert CONFIG_TEGRA_GPIO to Kconfig

Future chips will contain different GPIO HW. This change will enable
future SoC support to select the appropriate GPIO driver for their HW,
in a future-looking fashion, using Kconfig.

TEGRA_GPIO is not simply selected by TEGRA_COMMON (even though all
current Tegra chips used this GPIO HW) to simplify the later addition
of support for Tegra SoCs that use different GPIO HW.

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