dt-bindings: mailbox: Correct example indentation

DTS example in the bindings should be indented with 2- or 4-spaces, so
correct a mixture of different styles to keep consistent 4-spaces.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Sven Peter <sven@kernel.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>