gpio: Update Kconfig text for GPIO_SYSFS

This feature is deprecated, it is helpful to inform users about
this. I'm resisting the temptation to add "depends on BROKEN"
to this, but saving that for later.

Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>