gpio: export add/remove lookup table functions

For hot-pluggable devices adding GPIOs dynamically we need to
assemble and add the gpio lookup tables at probe time in modules,
so that requesting these GPIOs in attached drivers can work.
Export lookup table functions for modules.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>