SSM260x also supports 16kHz with external master clocks of 12.000MHz,
12.288MHz and 18.432MHz.
Add matching coefficients, update constraints and announced rates.
Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>