dt-bindings: arm: qcom: Document SDX55 platform and boards

Document the SDX55 platform binding and also the boards using it.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20201126083138.47047-2-manivannan.sadhasivam@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>