misc: atmel-ssc: register as sound DAI if #sound-dai-cells is present

The SSC is currently not usable with the ASoC simple-audio-card, as
every SSC audio user has to build a platform driver that may do as
little as calling atmel_ssc_set_audio/atmel_ssc_put_audio (which
allocates the SSC and registers a DAI with the ASoC subsystem).

So, have that happen automatically, if the #sound-dai-cells property
is present in devicetree, which it has to be anyway for simple audio
card to work.

Signed-off-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>