Platform/Sgi: Low Power Idle States for RD-V1 quad-chip platform

RD-V1 quad-chip platform supports two LPI states, LPI1 (Standby WFI) and
LPI3 (Power-down). Add idle support for RD-V1 quad-chip platform.

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>