ASoC: msm: add the Loopback MCLK mixer control

In 8916, codec loopback cannot work due to the MCLK being disabled.
As codec loopback only happens during development test and
MCLK enabling/disabling is tied to sessions in normal use case,
add the Loopback MCLK mixer control to enable/disable the MCLK
dynamically via tinymix command along with other codec loopback tinymix
commands, while ensuring the SSR can work fine in normal use cases

CRs-Fixed: 664068
Change-Id: I5098496973c6142950413b64dc7213176dd27349
Signed-off-by: Walter Yang <yandongy@codeaurora.org>