dt-bindings: mailbox: qcom-ipcc: simplify the example

Consumer examples in the bindings of resource providers are trivial,
useless and duplicating code.  Additionally the incomplete qcom,smp2p
example triggers DT schema warnings.

Cleanup the example by removing the consumer part and fixing the
indentation to DT schema convention.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>