msm: dt-binding: add snapshot of audio external clock header file for Kona

This is a snapshot of the external clock header file as of
msm-4.14 commit 'Ib9ccac4d28bb21e197cd3ea1563dce2601a0699e'
(add audio external clock header file for SDM855).

Change-Id: Ib9ccac4d28bb21e197cd3ea1563dce2601a0699e
Signed-off-by: Meng Wang <mwang@codeaurora.org>