dt-bindings: msm: add snapshot of msm_11ad doc

Add snapshot of wil6210.txt, the documentation for the
msm_11ad driver bindings.
The snapshot is matching commit c8250f7c
(msm_11ad: support voltage GPIO connected to 11ad card)
from msm-4.14 branch.

Change-Id: I6354c4e3be5daf50c453c3810e6bbd019580f427
Signed-off-by: Lior David <liord@codeaurora.org>