media: vicodec: Register another node for stateless decoder

Add stateless decoder instance field to the dev struct and
register another node for the statelsess decoder.
The stateless API for the node will be implemented in further patches.

Signed-off-by: Dafna Hirschfeld <dafna3@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix typo: videdev-stateless-dec -> stateless-decoder]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>