dt-bindings: mailbox: Drop consumers example DTS

Providers DTS examples should not contain consumer nodes, because they
are completely redundant, obvious (defined in common schema) and add
unnecessary bloat.  Drop consumer examples and unneeded node labels.

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