Platform/Sgi: Low Power Idle States for RD-N2

RD-N2 platform supports two LPI states, LPI1 (Standby WFI) and LPI3
(Power-down). The cluster supports LPI2 (Power-down) state. The LPI
implementation also supports combined power state for core and cluster.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Thomas Abraham <thomas.abraham@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>