Platform/Sgi: Fix ACPI Low Power Idle states for SGI575

Fix the ACPI _LPI object's control method for SGI-575 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: Id31a850638c6ba85da4e70869a43badf80e893ce
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>