ASoC: Add card registration API

ASoC v2 allows cards, codecs and platforms to instantiate separately,
with the overall ASoC device only being instantiated once all the
required components have registered. As part of backporting Liam's work
introduce an initial version of the card registration functions. At
present these do nothing active and are internal only, they will be
exposed to machine drivers after further backporting.  Adding this now
allows the datastructures used for dynamic card instantiation to be
built up gradually.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>