ACPI: HED: Install Notify() handler directly

Modify the ACPI HED driver to install its own Notify() handler directly
instead of providing an ACPI driver .notify() callback.

This will allow the ACPI driver .notify() callback to be eliminated and
it will allow the HED driver to be switched over to a platform one in
the future.

Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
[ rjw: Subject and changelog edits, whitespace adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>