dt-bindings: firmware: fsl,scu: Mark multi-channel MU layouts as deprecated

dt-bindings: firmware: fsl,scu: Mark multi-channel MU layouts as deprecated

The SCU MU driver has already supported the simple and efficient single-TX
and single-RX channel layout since 2021. The older multi-channel MU
configurations (tx0..tx3 and rx0..rx3) are less efficient in practice and
not needed.

Mark these legacy mbox-names and mboxes tuple layouts as deprecated in the
binding schema. The driver continues to support them for backward
compatibility in case firmware publishes the legacy properties.

The example section is updated accordingly to demonstrate the recommended
layout.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260127-scu-v2-1-03f3aaa56e1b@nxp.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>