Documentation: Add device tree bindings for msm-id and board-id

Add DT bindings for msm-id and board-id which are used to identify the
MSM chipsets and boards. This is a snapshot as of msm-4.14 commit
<4ffaab25>.

Change-Id: I10fa43959219a9135a289f9ce53cfa4146844898
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>