devicetree: bindings: Document qcom board compatible format

Some qcom based bootloaders identify the dtb blob based on a set
of device properties like SoC, platform, PMIC, and revisions of
those components. In downstream kernels, these values are added
to the different component dtsi files (i.e. pmic dtsi file, SoC
dtsi file, board dtsi file, etc.) via qcom specific DT
properties. The dtb files are parsed by a program called dtbTool
that picks out these properties and creates a table of contents
binary blob with the property information and some offsets into
the concatenation of all the dtbs (termed a QCDT image).

The suggestion is to do this via the board compatible string
instead, because these qcom specific properties are never used by
the kernel. Add a document describing the format of the
compatible string that encodes all this information that's
currently encoded in the qcom,{msm-id,board-id,pmic-id}
properties in downstream devicetrees. Future bootloaders may be
updated to look at the compatible field instead of looking for
the table of contents image. For non-updateable bootloaders, a
new dtbTool program will parse the compatible string and generate
a QCDT image from it.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>