arm64: zynqmp: Add idle state for ZynqMP

Added the idle-states node to describe zynqmp idle states. Only
cpu-sleep-0 idle state is added in this patch. References to the
idle-states node are added in all CPU nodes. Time values: entry/exit
latencies and min-residency, needs to be tuned. arm,psci-suspend-param
is selected to comply with PSCIv1.0 and Extended StateID format.

Signed-off-by: Stefan Krsmanovic <stefan.krsmanovic@aggios.com>
Acked-by: Will Wong <willw@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>