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>

Author: Bartosz Golaszewski

MAINTAINERS

@@ -8791,6 +8791,7 @@ F:	include/linux/gpio/regmap.h
 GPIO SUBSYSTEM
 M:	Linus Walleij <linus.walleij@linaro.org>
 M:	Bartosz Golaszewski <brgl@bgdev.pl>
+R:	Andy Shevchenko <andy@kernel.org>
 L:	linux-gpio@vger.kernel.org
 S:	Maintained
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/brgl/linux.git