[media] rc: ir-rc6-decoder: Add encode capability

Add the capability to encode RC-6 and RC-6A scancodes as raw events.

The Manchester modulation helper is used several times with various
timings so that RC-6 header preamble, the header, header trailing bit
and the data itself can be modulated correctly.

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