ACPI / processor_idle: Add support for Low Power Idle(LPI) states

ACPI 6.0 introduced an optional object _LPI that provides an alternate
method to describe Low Power Idle states. It defines the local power
states for each node in a hierarchical processor topology. The OSPM can
use _LPI object to select a local power state for each level of processor
hierarchy in the system. They used to produce a composite power state
request that is presented to the platform by the OSPM.

Since multiple processors affect the idle state for any non-leaf hierarchy
node, coordination of idle state requests between the processors is
required. ACPI supports two different coordination schemes: Platform
coordinated and  OS initiated.

This patch adds initial support for Platform coordination scheme of LPI.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>