msm: clock-8960/8660/9615: Allow PCM to source from external input

The pcm clock can source from either the internal mn counter or
the external codec input from a gpio. Support setting the rate of
the pcm clock to zero to support the external codec input. A
non-zero rate signifies use of the mn counter.

Change-Id: I6831dfea5f8fec6ee9330342edb7dcaeff4b33d4
CRs-fixed: 352645
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>