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>

MAINTAINERS

@@ -6299,6 +6299,7 @@ F:	tools/testing/selftests/gpio/
 
 GPIO SUBSYSTEM
 M:	Linus Walleij <linus.walleij@linaro.org>
+M:	Bartosz Golaszewski <bgolaszewski@baylibre.com>
 L:	linux-gpio@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git
 S:	Maintained