MAINTAINERS: update GPIO maintainers entry

Add Alexandre Courbot as co-maintainer of the GPIO subsystem.
Provide a pointer to the GPIO GIT tree. Update the documentation
file path. Move around to put people and git tree on top.

Acked-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>