docs: gpio: Add GPIO Aggregator documentation

Document the GPIO Aggregator, and the two typical use-cases.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20200511145257.22970-6-geert+renesas@glider.be


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