ACPI: Implement a generic FFH Opregion handler

This registers the FFH OpRegion handler before ACPI tables are
loaded. The platform support for the same is checked via Platform-Wide
OSPM Capabilities(OSC) before registering the OpRegion handler.

It relies on the special context data passed to offset and the length.
However the interpretation of the values is platform/architecture
specific. This generic handler just passed all the information to
the platform/architecture specific callback. It also implements the
default callbacks which return as not supported.

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