dt-bindings: arm: qcom: Document SM8250 SoC and boards

Document the SM8250 SoC binding and also the boards using it.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20200904063637.28632-2-manivannan.sadhasivam@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>