gpio: Export gpiochip_irq_{map,unmap}()

Export these functions so that drivers can explicitly use these when
setting up their IRQ domain.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>