dt-bindings: mailbox: ti,secure-proxy: Convert to json schema

Convert the ti,secure-proxy to yaml for better checks and documentation.
Differences being mostly in the examples:
- Dropped the example usage of mailbox client, it is better done in
  tisci node definition.
- Switched reg usage for address-cells and size-cells 1 - aligned with
  schema checks as well.
- included header in example for a buildable example.

While at this, lets make sure to support upto 100 rx threads even though
typically upto 1 threads is practically in use.

NOTE: The following checkpatch warning is generated since we do include the header
in the example, but this is a false positive warning.
  WARNING: DT binding docs and includes should be a separate patch. See:
  Documentation/devicetree/bindings/submitting-patches.rst

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>