dt-bindings: msm: Document "register collection" in qcom-ipa node

Add documentation for "register collection" attributes in the qcom-ipa
node.

Change-Id: I472ade3f80dda77bdb46cb22bb71598087679f6b
CRs-Fixed: 2401867
Signed-off-by: Jennifer L. Zenner <jzenner@codeaurora.org>