tests/drivers/gpio/gpio_reserved_ranges: Fix overlays

Correct overlays names.
In 982425f
this test was changed from using native_posix to native_sim,
but the overlay file renaming slipped thru the cracks.
Due to the test DT filter, the test just was not run since then,
and we did not realize.
Let's fix it now so the test can run again.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar