MAINTAINERS: add Andy Shevchenko as reviewer for the GPIO subsystem

Andy has been a de-facto reviewer for all things GPIO for a long time so
let's make it official.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>