The qcom,ipc-N properties are essentially providing a reference to a
mailbox, so allow using the mboxes property to do the same in a more
structured way.

Since multiple SMSM hosts are supported, we need to be able to provide
the correct mailbox for each host. The old qcom,ipc-N properties map to
the mboxes property by index, starting at 0 since that's a valid SMSM
host also.

Mark the older qcom,ipc-N as deprecated and update the example with
mboxes.
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240606-smsm-mbox-v2-1-8abe6b5f01da@z3ntu.xyzSigned-off-by: Bjorn Andersson <andersson@kernel.org>