Platform/Sgi: ACPI CPPC support for RD-N2

Enable ACPI CPPC mechanism for RD-N2 as defined by the ACPI
specification. The implementation uses AMU registers accessible as
Fixed-feature Hardware (FFixedHW) for monitoring the performance.
Non-secure SCMI fastchannels are used to communicate with SCP to set
the desired performance. RD-N2 platform does not support CPPC revision
1 and below. So update the _OSC method to let OSPM know about this fact.

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>