Deprecate the qcom,ipc way of accessing the mailbox in favor of the
'mboxes' property.

Update the example to use mboxes.
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Link: https://lore.kernel.org/r/20240425-qcom-ipc-deprecate-v1-2-a8d8034253ea@z3ntu.xyzSigned-off-by: Bjorn Andersson <andersson@kernel.org>