msm: clk: clock-controller: Add snapshot of DT binding documentation

Add the DT binding documentation for the MSM clock controller, taken
from msm-3.14 as of commit dbdb6776f (Merge "Update target name
references for 8976")

Change-Id: I5cbbd0c1a922580e39fd8214d3c4aff835384b1b
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>