gpiolib: Document new gpio_chip.init_valid_mask field

A new field init_valid_mask was added to struct gpio_chip, but it was
not documented.

Fixes: f8ec92a9 ("gpiolib: Add init_valid_mask exported function")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190701142650.25122-1-geert+renesas@glider.be


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