cpuidle: psci: Move the per CPU variable domain_state to a struct

To prepare to extend the per CPU variable domain_state to include more
data, let's move it into a struct. A subsequent change will add the new
data. This change have no intended functional impact.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250314100103.1294715-3-ulf.hansson@linaro.org