Platform/Sgi: ACPI CPPC support for RD-N2

Enable ACPI CPPC mechanism for RD-N2. 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.

Change-Id: I6ab4f4e67511b7c2b3e800bd28ceb5559a78f816
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>