MAINTAINERS: add content regex for gpio-regmap

Add a glob to get patches of the users of gpio-regmap, too.

Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>