gpio: mockup: Adjust documentation to the code

First of all one of the parameter missed 'mockup' in its name,
Second, the semantics of the integer pairs depends on the sign
of the base (the first value in the pair).

Update documentation to reflect the real code behaviour.

Fixes: 2fd1abe9

 ("Documentation: gpio: add documentation for gpio-mockup")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>