ACPI: tiny-power-button: Eliminate the driver notify callback

Rework the ACPI tiny-power-button driver to install a notify handler or
a fixed event handler for the device it binds to by itself and drop its
notify callback.

This will allow acpi_device_install_notify_handler() and
acpi_device_remove_notify_handler() to be simplified going forward.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>