ACPI / x86: Add PWM2 on the Xiaomi Mi Pad 2 to the always_present list

The Xiaomi Mi Pad 2 has backlit LEDs behind the capacitive menu, home
and back buttons below the screen which are controlled by the PWM2
controller of the CHT SoC. This PWM2 controller gets hidden by the
firmware, add it to the always_present_ids table so that we can use
the PWM controller to control the backlighting of the buttons.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>