[TF-M] Some comments on twincpu mailbox design

Hi Danny,

Could you please check my comments on [Mailbox design](https://developer.trustedfirmware.org/w/tf_m/design/twin-cpu/mailbox-hal/) in below?

I’d like to discuss about some high-level designs at the very first time. We can discuss about details later. 😊

1. As designed in Concepts section, secure core has a single dispatcher thread, which is the main thread on the secure core.

AFAIK, there is no such main thread in current TF-M Inter-Process Communication model implementation. Do you mind taking a look at the section "Interaction in thread model" in our [TF-M presentation](https://events.linuxfoundation.org/wp-content/uploads/2017/12/OpenIoT-Summit...

On the other hand, implementing a dedicated thread in TF-M to handling the mailbox events can be a solution. But it might also bring some difficulties in implementation. I'd like to discuss in detail with Transport Layer together.

2. I like the concept of mailbox event. One of the key points is, after the information carried via mailbox arrives in TF-M, how to deliver it to TF-M core. We can have further discussions on the details.

3. PSA Firmware Framework specifies the parameters and return value for PSA Client APIs. Do you think if it is necessary to cover the mailbox message structure containing information of parameters and return value in mailbox design?

Thank you.

Best regards, Hu Ziji