MAINTAINERS: remove self from GPIO maintainers

I have not been able to dedicate time to the GPIO subsystem since quite
some time, and I don't see the situation improving in the near future.
Update the maintainers list to reflect this unfortunate fact.

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