lib: utils: Add fdt_add_cpu_idle_states() helper function

Since the availability and latency properties of CPU idle states depend
on the specific SBI HSM implementation, it is appropriate that the idle
states are added to the devicetree at runtime by that implementation.

This helper function adds a platform-provided array of idle states to
the devicetree, following the SBI idle state binding.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>