platform/x86: dell/dell-rbtn: Register ACPI notify handler directly

platform/x86: dell/dell-rbtn: Register ACPI notify handler directly

To facilitate subsequent conversion of the driver to a platform one,
make it install an ACPI notify handler directly instead of using
a .notify() callback in struct acpi_driver.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/9591832.CDJkKcVGEf@rafael.j.wysocki
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>