[media] serio.h: add SERIO_RAINSHADOW_CEC ID

Add a new serio ID for the RainShadow Tech USB HDMI CEC adapter.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>