ASoC: fsl: Move fsl-ssi binding doc to sound/

fsl-ssi was located in powerpc/fsl/ssi.txt. This is no powerpc specific
device, so it should be moved to sound/ as it connects to differen audio
codecs.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>