lib: utils/gpio: respect flag GPIO_FLAG_ACTIVE_LOW

"gpio-poweroff" and "gpio-restart" always set gpio to high to
active the function, but some chips need a low signal to active.
Fortunately, it can be achieved by setting GPIO_FLAG_ACTIVE_LOW
for the gpio. Implement this flag support for the gpio library
so the gpio reset can function well.

Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
Reviewed-by: Anup Patel <anup@brainfault.org>