MAINTAINERS: Add gpio regmap section

Add myself as a reviewer for the gpio regmap.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20200528145845.31436-4-michael@walle.cc


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>