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: NMarkus Pargmann <mpa@pengutronix.de>
Tested-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NMark Brown <broonie@linaro.org>