cpuidle: psci: Split psci_dt_cpu_init_idle()

To make the code a bit more readable, let's move the OSI specific
initialization out of the psci_dt_cpu_init_idle() and into a separate
function.

Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>