gpio: fix doc string for devm_gpiochip_add_data() to not talk about irq_chip

The function is about adding a gpio_chip so dev has to belong to this
one. Fix wording to be more grammatically correct (but attention, I'm
not a native speaker).

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>