dt-bindings: arm: qcom: Document SM8450 SoC and boards

Document the SM8450 SoC binding and also the boards using it.

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211201072745.3969077-4-vkoul@kernel.org