drivers: firmware: psci: Simplify error path of psci_dt_init()

Instead of having each PSCI init function taking care of the
of_node_put(), deal with that from psci_dt_init(), as this enables
a bit simpler error path for each PSCI init function.

Co-developed-by: Lina Iyer <lina.iyer@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>