gpiolib: export gpiochip_get_desc

The function was currently used internal by the gpiolib. Since commit
56cc3af4 ("pinctrl: da9062: add driver support") it is also used by
drivers so we need to export the symbol.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20200225093102.10964-1-m.felsch@pengutronix.de


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>