tegra: Allow boards to perform early GPIO setup

The new gpio_early_init() function, which does nothing by default, can
be overridden by boards to configure GPIOs at an early stage.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>