Platform/Sgi: Low Power Idle States for RD-N1-Edge dual-chip

RD-N1-Edge platform in multi chip configuration supports 2 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>