ASoC: add DAI platform ssi driver for MXC

This adds support for DAI platform for the SSI present in MXC platforms.

It currently does not support i.MX3, the only thing necessary to do
this is to export DMA data for i.MX3 interface which I haven't done
because I don't have a i.MX3 based board available.

It has been tested on i.MX27 board.

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