dt-bindings: remoteproc: qcom,q6v5: Move MSM8916 to schema

qcom,q6v5.txt covers multiple SoCs with quite different binding
requirements. Converting this into one DT schema would require
several if statements, making the DT schema overall harder to
read and understand.

To avoid this, follow the example of SC7180/SC7280 and split
"qcom,msm8916-mss-pil" (and the equivalent deprecated "qcom,q6v5-pil"
compatible) into a separate DT schema. The schema is somewhat based
on the one for SC7180/SC7280 but adjusted for the old platforms.

Compared to the old plain text bindings, add missing documentation for
the "bam-dmux" subnode and recommend one particular approach to specify
the MBA/MPSS "memory-region" (the other one is marked as deprecated).

Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220908182433.466908-3-stephan.gerhold@kernkonzept.com