gpio: tegra: Use platform_get_irq()

Instead of using platform_get_resource() and getting the interrupts from
the resource, use platform_get_irq() which is slightly easier to use and
covers some special cases that the former doesn't.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>