gpiolib: add a common prefix to GPIO descriptor flags

gpiolib: add a common prefix to GPIO descriptor flags

While these flags are private within drivers/gpio/, when looking at the
code, it's not really clear they are GPIO-specific. Since these are GPIO
descriptor flags, prepend their names with a common "GPIOD" prefix.

While at it: update the flags' docs: make spelling consistent, correct
outdated information, etc.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250909-rename-gpio-flags-v1-1-bda208a40856@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>