msm: dt-binding: add audio external clock header file for SDM855

AUDIO_PMI_CLK, AUDIO_PMIC_LNBB_CLK and AUDIO_LPASS_MCLK can be external
clock provider of wcd codec. Add header file to indicate index for these
three external clock provider.

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