gpio: tegra186: Allow building on Tegra194-only configurations

The driver is compatible with both Tegra186 and Tegra194, but currently
it cannot be selected if only Tegra194 support is enabled. Allow builds
with only Tegra194 support enabled to select this driver.

While at it, select this driver by default on Tegra194 builds because it
is an essential part of the system.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20191213130034.219227-1-thierry.reding@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>