gpiolib: add gpiod_is_single_ended() helper

gpiolib: add gpiod_is_single_ended() helper

The direction of a single-ended (open-drain or open-source) GPIO line
cannot always be reliably determined by reading hardware registers.
In true open-drain implementations, the "high" state is achieved by
entering a high-impedance mode, which many hardware controllers report
as "input" even if the software intends to use it as an output.

This creates issues for consumer drivers (like I2C) that rely on
gpiod_get_direction() to decide if a line can be driven.

Introduce gpiod_is_single_ended() to allow consumers to check the
software configuration (GPIO_FLAG_OPEN_DRAIN/GPIO_FLAG_OPEN_SOURCE) of
a descriptor. This provides a robust way to identify lines that are
capable of being driven, regardless of their instantaneous hardware state.

Signed-off-by: Jie Li <jie.i.li@nokia.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260511113726.49041-2-jie.i.li@nokia.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>