gpio: sandbox: Use a separate flag for the value

gpio: sandbox: Use a separate flag for the value

At present with the sandbox GPIO driver it is not possible to change the
value of GPIOD_IS_OUT_ACTIVE unless the GPIO is an output. This makes it
hard to test changing the flags since we need to be aware of the internal
workings of the driver.

The feature is designed to aid testing.

Split this feature out into a separate sandbox-specific flag, so that the
flags can change unimpeded. This will make it easier to allow updating the
flags in a future patch.

Signed-off-by: Simon Glass <sjg@chromium.org>