dt-bindings: arm: qcom: Document SDX65 platform and boards

Document the SDX65 platform binding and also the boards using it.

Signed-off-by: Vamsi Krishna Lanka <quic_vamslank@quicinc.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/500c75eb1bdc37ad66dc2ebd51ad21a211d9ed27.1626987430.git.quic_vamslank@quicinc.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>