dt-bindings: mailbox : arm,mhu: Use examples with matching schema

dt-bindings: mailbox : arm,mhu: Use examples with matching schema

Currently the example provided in arm,mhu schema complains as below:

    Documentation/devicetree/bindings/mailbox/arm,mhu.example.dt.yaml :0:0:
    /example-0/soc/scb@2e000000: failed to match any schema with compatible:
    ['fujitsu,mb86s70-scb-1.0']

Fix the same using examples based on Juno platform. The old SCPI firmware
used MHU with standard 32-bit data transfer protocol while the new SCMI
firmware uses MHU and expects to be used in doorbell mode. Update example
with SCPI and SCMI firmware nodes to demonstrate both 32-bit data transfer
and doorbell mode of MHU operations

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20210604205710.1944363-6-sudeep.holla@arm.com
Signed-off-by: Rob Herring <robh@kernel.org>