[media] rc: rc-ir-raw: Add pulse-distance modulation helper

Add IR encoding helper for pulse-distance modulation as used by the NEC
protocol.

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