MAINTAINERS: add more keywords for the GPIO subsystem entry

MAINTAINERS: add more keywords for the GPIO subsystem entry

Add GPIOD_FLAGS_BIT_NONEXCLUSIVE and devm_gpiod_unhinge as keywords to
the GPIO entry so that we get notified if anybody tries to use these
deprecated symbols. We'll drop them from here once we remove them from
the kernel.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250401-gpio-todo-remove-nonexclusive-v2-3-7c1380797b0d@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>