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>

dt-bindings: mailbox : arm,mhuv2: Add bindings

This patch adds device tree binding for ARM Message Handling Unit (MHU)
controller version 2.

Based on earlier work by Morten Borup Petersen.

Reviewed-by: Rob Herring <robh@kernel.org>
Co-developed-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Tushar Khandelwal <tushar.khandelwal@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>