[media] rc: rc-ir-raw: Add scancode encoder callback

Add a callback to raw ir handlers for encoding and modulating a scancode
to a set of raw events. This could be used for transmit, or for
converting a wakeup scancode to a form that is more suitable for raw
hardware wake up filters.

Signed-off-by: James Hogan <james@albanarts.com>
Signed-off-by: Antti Seppälä <a.seppala@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Cc: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>