ASoC: simple-card: add support for aux devices

Add device tree property to define auxiliary devices to be added to
simle-audio-card.

Together with proper audio routing definition, this allows to use
simple-card in setups where separate amplifier chip is connected to
codec's output.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Mark Brown <broonie@kernel.org>