gpiolib: Clarify use of non-sleeping functions

Obviously functions that are safe to be called from atomic contexts, can
be called from non-atomic contexts, too.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20190701142809.25308-1-geert+renesas@glider.be


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