gpio: sim: allow to mark simulated lines as invalid

Add a new line-level, boolean property to the gpio-sim configfs
interface called 'valid'. It's set by default and the user can unset it
to make the line be included in the standard `gpio-reserved-ranges`
property when the chip is registered with GPIO core. This allows users
to specify which lines should not be available for requesting as GPIOs.

Link: https://lore.kernel.org/r/20250630130358.40352-1-brgl@bgdev.pl


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>