ASoC: Change the PCM runtime array to a list

Currently the number of DAI links is statically defined by the machine
driver at build time using an array. This makes it difficult to shrink/
grow the number of DAI links at runtime in order to reflect any changes
in topology.

We can change the DAI link array in the core to a list so that PCMs and
FE DAI links can be added and deleted at runtime to reflect changes in
use case and DSP topology. The machine driver can still register DAI links
as an array.

As the 1st step, this patch change the PCM runtime array to a list. A new
PCM runtime is added to the list when a DAI link is bound successfully.

Later patches will further implement the DAI link list.

More:
- define snd_soc_new/free_pcm_runtime() to create/free a runtime.
- define soc_add_pcm_runtime() to add a runtime to the rtd list.
- define soc_remove_pcm_runtimes() to clean up the runtime list.

- traverse the rtd list to probe the link components and dais.

- Add a field "num" to PCM runtime struct, used to specify the device
  number when creating the pcm device, and for a soc card to access
  its dai_props array.

- The following 3rd party machine/platform drivers iterate the rtd list
  to check the runtimes:
  sound/soc/intel/atom/sst-mfld-platform-pcm.c
  sound/soc/intel/boards/cht_bsw_rt5645.c
  sound/soc/intel/boards/cht_bsw_rt5672.c
  sound/soc/intel/boards/cht_bsw_max98090_ti.c

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>