gpio: Add use guidance documentation

The text in this new document is a response to recurring questions
about the GPIO in-kernel API vs the userspace ABI. When do you use
one or the other? It can be a bit intuitive, but I tried to sum it
all up.

Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200108001712.47500-1-linus.walleij@linaro.org


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