gpio: mm-lantiq: update kernel docs

gpio: mm-lantiq: update kernel docs

Update kernel docs which are now outdated following the conversion to
using the modern GPIO provider API.

Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Fixes: 8d0d46da40c8 ("gpio: mm-lantiq: Drop legacy-of-mm-gpiochip.h header from GPIO driver")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510290348.IpSNHCxr-lkp@intel.com/
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20251029091138.7995-1-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>