Platform/Sgi: Low Power Idle states for RD-V1 platform

RD-V1 platform supports 2 LPI states, LPI1 (Standby WFI) and LPI3
(Power-down). Add idle support for RD-V1 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>