arm64: tegra: Add gpio-keys nodes for Smaug

Add gpio-keys nodes for the volumn controls, lid switch, tablet mode and
power button.

Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
[treding@nvidia.com: use symbolic names for input types and codes]
[treding@nvidia.com: use wakeup-source instead of gpio-key,wakeup]
Signed-off-by: Thierry Reding <treding@nvidia.com>