platform/x86: x86-android-tablets: Add a comment about x86_android_tablet_get_gpiod()

Add a comment explaining why the special x86_android_tablet_get_gpiod()
helper is necessary for some of the GPIOs used in the x86-android-tablets;
and that for normal GPIO lookups this example should not be followed.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20230911210928.264908-1-hdegoede@redhat.com