gpio: introduce utilities for synchronous fake device creation

Both gpio-sim and gpio-virtuser share a mechanism to instantiate a
platform device, wait for probe completion, and retrieve the probe
success or error status synchronously. With gpio-aggregator planned to
adopt this approach for its configfs interface, it's time to factor
out the common code.

Add dev-sync-probe.[ch] to house helper functions used by all such
implementations.

No functional change.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250221133501.2203897-2-koichiro.den@canonical.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>