ASoC: msm: qdsp6: add support to query RMS during recording

Recorder app requires RMS value of the recording when using
tunnel mode AMR-WB encoding. Add mixer control so app can query
the RMS value from DSP

Change-Id: I84127ad35b916d257b0edee260141c34b92742f2
Signed-off-by: Jitendra Singh Naruka <jnaruk@codeaurora.org>
Signed-off-by: flar2 <asegaert@gmail.com>