MAINTAINERS: add myself as reviewer for gpio-mockup

The user space libgpiod project is now tightly coupled with the
gpio-mockup module, so let me know when any changes to it are being
proposed.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>