[media] rainshadow-cec: new RainShadow Tech HDMI CEC driver

This driver supports the RainShadow Tech USB HDMI CEC adapter.

See: http://rainshadowtech.com/HdmiCecUsb.html



Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>