MAINTAINERS: add myself as co-maintainer of gpiolib

As discussed with Linus Walleij - I'm adding myself as
the co-maintainer.

Cc: Benoît Cousson <bcousson@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>