gpio: virtuser: convert to use dev-sync-probe utilities

Update gpio-virtuser to use the new dev-sync-probe helper functions for
synchronized platform device creation, reducing code duplication.

No functional change.

Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Link: https://lore.kernel.org/r/20250221133501.2203897-4-koichiro.den@canonical.com


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