gpio: mockup: use KBUILD_MODNAME

Drop the definition for the driver name. Let's use KBUILD_MODNAME for
the log format and use the "gpio-mockup" value directly in the only
place where it's relevant: in the name of the device.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>