MAINTAINERS: Add GPIO Aggregator section

Add a maintainership section for the GPIO Aggregator, covering
documentation and driver source code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20200511145257.22970-7-geert+renesas@glider.be


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