dt-bindings: mailbox : arm,mhuv2: Use example with matching schema

dt-bindings: mailbox : arm,mhuv2: Use example with matching schema

Currently the example provided in arm,mhuv2 schema complains as below:

    Documentation/devicetree/bindings/mailbox/arm,mhuv2.example.dt.yaml :0:0:
    /example-0/soc/scb@2e000000: failed to match any schema with compatible:
    ['fujitsu,mb86s70-scb-1.0']

Fix it by using an example with a matching schema that makes use of 4
mailboxes that is well suited to demonstrate Rx and Tx channels with both
doorbell and data transfer protocols.

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/20210604205710.1944363-7-sudeep.holla@arm.com
Signed-off-by: Rob Herring <robh@kernel.org>