dt-bindings: mailbox : arm,mhu: Fix arm,scpi example used here

dt-bindings: mailbox : arm,mhu: Fix arm,scpi example used here

Once the arm,scpi binding is converted to YAML format, the following
errors will be seen when doing `make DT_CHECKER_FLAGS=-m dt_binding_check`

From schema: Documentation/devicetree/bindings/firmware/arm,scpi.yaml
Documentation/devicetree/bindings/mailbox/arm,mhu.example.dt.yaml:
	scpi@2f000000: $nodename:0: 'scpi' was expected
Documentation/devicetree/bindings/mailbox/arm,mhu.example.dt.yaml:
	scpi@2f000000: reg: [[0, 788529152, 0, 512]] is not of type 'object'
Documentation/devicetree/bindings/mailbox/arm,mhu.example.dt.yaml:
	scpi@2f000000: 'shmem' is a required property

Fix those error following the SCPI bindings.

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/20210604205710.1944363-2-sudeep.holla@arm.com
Signed-off-by: Rob Herring <robh@kernel.org>