MAINTAINERS: gpio-regmap: make myself a maintainer of it

When I've upstreamed the gpio-regmap driver, I didn't have that much
experience with kernel maintenance, so I've just added myself as a
reviewer. I've gained quite some experience, so I'd like to step up
as a maintainer for it.

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>