ASoC: fsl-ssi: Add imx51-ssi and of_device_id matching

There is a small but significant difference between imx21-ssi and
imx51-ssi and above. imx21-ssi does not allow online reconfiguration of
some registers. They have to be configured at the beginning.

imx51-ssi has to reconfigure the SSI unit while it is running. Otherwise
it would not be possible to have two streams in parallel. The new SDMA
unit in imx51 and above has to be configured before the first DMA
request arrives. Therefor we need to setup TDMAE/RDMAE just before
starting the stream.

This patch introduces distinct imx51-ssi as compatible and adds
of_device_id matching in the probe function.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>