dt-bindings: mailbox: qcom-ipcc: add missing properties into example

These missing required properties are needed for
smp2p binding reference checks.

Also includes cosmetic change to the example formatting.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David Heidelberg <david@ixit.cz>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220426101837.16201-2-david@ixit.cz