ACPI / PMIC: AXP288: support virtual GPIO in ACPI table

The same virtual GPIO strategy is also used for the AXP288 PMIC in that
various control methods that are used to do power rail handling and
sensor reading/setting will touch GPIO fields defined under the PMIC
device. The GPIO fileds are only defined by the ACPI code while the
actual hardware doesn't really have a GPIO controller, but to make those
control method execution succeed, we have to install a GPIO handler for
the PMIC device handle. Since we do not need the virtual GPIO strategy,
we can simply do nothing in that handler.

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