Platform/Sgi: Fix ACPI Low Power Idle states for RD-N1-Edge-X2

Fix the ACPI _LPI object's control method for RD-N1-Edge-X2 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: I3d46537a223875ae3073c4822ff5b7ded067e4fe
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>