dt-bindings: msm: Document "platform-type" attribute in qcom-ipa node

Add documentation for "platform-type" attribute in qcom-ipa node.
Platform type can be MDM or MSM or APQ.

CRs-Fixed: 2341003
Change-Id: Iffec7d3a8b63fc43942864d9b3146d93606d26ef
Signed-off-by: Ghanim Fodi <gfodi@codeaurora.org>