Platform/Sgi: Fix ACPI Low Power Idle states for RD-N1-Edge

Fix the ACPI _LPI object's control method for RD-N1-Edge 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.

Change-Id: I3377583a83aaf81327acd7b617c98c6247fd6513
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>