ASoC: Add SND_SOC_DAPM_PRE_POST_PMD event

Some systems codecs need to configure some registers before and after
powering down some of their part. As a convenience add a macro for that.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>