dt-bindings: arm: qcom: Document SDX75 platform and boards

Document the SDX75 platform binding and also the boards using it.

Signed-off-by: Rohit Agarwal <quic_rohiagar@quicinc.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/1686311438-24177-2-git-send-email-quic_rohiagar@quicinc.com