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>

MAINTAINERS

@@ -5964,6 +5964,7 @@ F:	drivers/media/rc/gpio-ir-tx.c
 
 GPIO MOCKUP DRIVER
 M:	Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
+R:	Bartosz Golaszewski <brgl@bgdev.pl>
 L:	linux-gpio@vger.kernel.org
 S:	Maintained
 F:	drivers/gpio/gpio-mockup.c