cpuidle: psci: Drop superfluous wrappers psci_dt_attach|detach_cpu()

To simplify the code, let's drop psci_dt_attach|detach_cpu() and use the
common dt_idle_attach|detach_cpu() directly instead.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20240228151139.2650258-1-ulf.hansson@linaro.org