dt-bindings: arm: qcom: document qcom,msm-id and qcom,board-id

The top level qcom,msm-id and qcom,board-id properties are utilized by
bootloaders on Qualcomm MSM platforms to determine which device tree
should be used and passed to the kernel.

The commit b32e592d3c28 ("devicetree: bindings: Document qcom board
compatible format") from 2015 was a consensus during discussion about
upstreaming qcom,msm-id and qcom,board-id fields.  There are however still
problems with that consensus:
1. It was reached 7 years ago but it turned out its implementation did
   not reach all possible products.

2. Initially additional tool (dtbTool) was needed for parsing these
   fields to create a QCDT image consisting of multiple DTBs, later the
   bootloaders were improved and they use these qcom,msm-id and
   qcom,board-id properties directly.

3. Extracting relevant information from the board compatible requires
   this additional tool (dtbTool), which makes the build process more
   complicated and not easily reproducible (DTBs are modified after the
   kernel build).

4. Some versions of Qualcomm bootloaders expect these properties even
   when booting with a single DTB.  The community is stuck with these
   bootloaders thus they require properties in the DTBs.

Since several upstreamed Qualcomm SoC-based boards require these
properties to properly boot and the properties are reportedly used by
bootloaders, document them along with the bindings header with constants
used by: bootloader, some DTS and socinfo driver.

Link: https://lore.kernel.org/r/[email protected]/
Co-developed-by: Kumar Gala <[email protected]>
Signed-off-by: Kumar Gala <[email protected]>
Signed-off-by: Krzysztof Kozlowski <[email protected]>
Reviewed-by: Dmitry Baryshkov <[email protected]>
Reviewed-by: Rob Herring <[email protected]>
Signed-off-by: Bjorn Andersson <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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