ASoC: msm8916-wcd-digital: Add sidetone support

This patch adds sidetone support via one of the 3 RX Mix paths
using IIR1 and IIR2.
IIR1 can be feed by any Decimators or RX paths, and IIRx can also be
looped back to RX mixers to provide sidetone functionality.
Two IIR filters are used for Side tone equalization and each filter
is 5 stage.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>