soc: nrf54h: gpd: use callback to fetch nrfs async result

Busy-waiting for the result of the nrfs service calls can stall, so
let's use a callback that flags a semaphore instead. Since the API is
supposed to be callable in the context of pre-kernel, fallback to
busy-wait on that scenario.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>