Documentation: Add snapshot of msm_ion.txt

Add a snapshot of the ion devicetree documentation as of:
commit ed1836956c80c200cc7a290643019fa18f76e72c
("msm: kgsl: Change GPU clock gating sequence for A4XX and A5XX")

Change-Id: Iaeb6cc33b9f054ca55e89eaeb2b9c295498c9798
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>