gpio: mockup: move the driver out of the IOMEM drivers section

The testing driver doesn't really depend on HAS_IOMEM. We may want to
build it for testing purposes on architectures not supporting IOMEM,
for example: on user-mode linux. Move it out of the "Memory Mapped GPIO
drivers" section.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>