MAINTAINERS: Change myself to a maintainer

MAINTAINERS: Change myself to a maintainer

After some off-list discussion with Marek Vasut and Geert Uytterhoeven
and finally a kx022a driver related discussion with Joe Perches
https://lore.kernel.org/lkml/92c3f72e60bc99bf4a21da259b4d78c1bdca447d.camel@perches.com/
it seems that my status as a reviewer has been wrong. I do look after
the ROHM/Kionix drivers I've authored and currently I am also paid to do
so as is reflected by the 'S: Supported'. According to Joe, the reviewer
entry in MAINTAINERS do not indicate such level of support and having a
reviewer supporting an IC is a contradiction.

Switch undersigned from a reviewer to a maintainer for IC drivers I am
taking care of.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>