platform/x86: acer-wmi: add automatic keyboard background light toggle key as KEY_LIGHTS_TOGGLE

Got a dmesg message on my AMD Renoir based Acer laptop:
"acer_wmi: Unknown key number - 0x84" when toggling keyboard
background light

Signed-off-by: Timo Witte <timo.witte@gmail.com>
Reviewed-by: "Lee, Chun-Yi" <jlee@suse.com>
Link: https://lore.kernel.org/r/20200804001423.36778-1-timo.witte@gmail.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>