gpio: mockup: add prefixes to the direction enum

All internal symbols except for the direction enum follow the same
convention and use the gpio_mockup prefix. Add the prefix to the
DIR_IN and DIR_OUT definitions as well for consistency across the
file.

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