ASoC: msm: Add support for DSP based DTMF generation

Add support to generate DTMF using DSP's audio front end(AFE).
Add DTMF platform driver, update the routing table to enable
routing of DTMF FE to the BEs in ASoC.

Change-Id: I485c052e940285ece664e07e7c73d08f03ea958a
Signed-off-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>