V4L/DVB: ir-nec-decoder: Add sysfs node to enable/disable per irrcv

With the help of raw_register/raw_unregister, adds a sysfs group
associated with the decoder, inside the corresponding irrcv node.

Writing 1 to nec_decoder/enabled enables the decoder, while
writing 0 disables it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>