gpio: MAINTAINERS: Add an entry for GPIO mockup driver

Add an entry for the GPIO mockup driver with myself as maintainer.

Signed-off-by: Bamvor Jian Zhang <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

MAINTAINERS

···
 S:	Maintained
 F:	drivers/input/touchscreen/goodix.c
 
+GPIO MOCKUP DRIVER
+M:	Bamvor Jian Zhang <[e-mail]>
+L:	[e-mail]
+S:	Maintained
+F:	drivers/gpio/gpio-mockup.c
+F:	tools/testing/selftests/gpio/
+
 GPIO SUBSYSTEM
 M:	Linus Walleij <[e-mail]>
 M:	Alexandre Courbot <[e-mail]>