Platform/Sgi: Fix ACPI Low Power Idle states for RD-N2-Cfg2

Fix the ACPI _LPI object's control method for RD-N2 platform. This
platform supports only the platform co-ordinated LPI. Accordingly,
correct the LPI Level ID value. Additionally, as this platform does not
support residency counter, clear the residency counter frequency from
_LPI object's control method. The RD-N2-Cfg2 platform is a direct
connect platform, hence this patch remove the cluster _LPI control
method from DSDT table.

Change-Id: I19e8aa6864f0a22b3ed3b52a840d0a3753c2e3b7
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>