dt-bindings: arm: qcom: Document SM8450 HDK boards

Document the SM8450 HDK board

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220203090031.3128702-1-vkoul@kernel.org