docs: kernel-parameters: Move gpio-mockup for alphabetic order

All other sections are ordered alphabetically so do the same for
gpio-mockup.

Fixes: 0f98dd1b

 ("gpio/mockup: add virtual gpio device")
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>