[TF-A] PSA calls with RSE comms and MHU

Hello,

I would like to use PSA calls for an application on STM32MP2 platform. I've seen the psa_call() API that I could use, in drivers/arm/rse/rse_comms.c.

The problem is that it is based on MHU mailbox, which I don't have on STM32MP2. So I was planning plan to disentangle RSE comms and MHU, having a new file in the same directory to manage MHU. This code should then be under a flag, I could re-use PLAT_MHU_VERSION. Setting it to 0, would mean no MHU.

AFAICT TF-M uses that split with a dedicated file: rse_comms_hal.c: Abstracts MHU message sending and receiving. (see https://tf-m-user-guide.trustedfirmware.org/platform/arm/rse/rse_comms.html).

Do you think this approach sounds right?

Thanks, Yann