cpuidle: psci: Fix regression leading to no genpd governor

While factoring out the PM domain related code from PSCI domain driver into
a set of library functions, a regression when initializing the genpds got
introduced. More precisely, we fail to assign a genpd governor, so let's
fix this.

Fixes: 9d976d67 ("cpuidle: Factor-out power domain related code from PSCI domain driver")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>