ALSA: pcm - add support for hostless audio

Allow some PCM devices to be hostless, i.e. there is no PCM data transferred
to or from the host CPU. This can be used to minimise power on systems since
the CPU can idle/sleep during the PCM device operation (e.g. a phone call
where the DAI is between a MODEM and DSP)

TODO: cleanup, look at adding a read/write blocker.

Singed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>