Documentation: gpio: Remove text about ARCH_NR_GPIOS

ARCH_NR_GPIOS have been removed, clean up the documentation.

After this patch, the only place when ARCH_NR_GPIOS remains is in
translations/zh_CN/gpio.txt and translations/zh_TW/gpio.txt.
I don't have the skills to update that, anyway those two files are
already out of sync as they are still mentionning ARCH_REQUIRE_GPIOLIB
which was removed by commit 65053e1a

 ("gpio: delete
ARCH_[WANTS_OPTIONAL|REQUIRE]_GPIOLIB")

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>