ASoC: msm: msm8994: add sample rate mixer control for SLIM0 TX back-end

SLIMBUS_0_TX back-end is always being fixed up to 48khz currently.
As part of effort to support true greater than 48khz recording,
expose mixer control to allow 48khz, 96khz, and 19khz sample rate
selection.

Signed-off-by: Patrick Lai <plai@codeaurora.org>
Change-Id: Ia202ef066a394924eb9574ef4e3115611493b84a