scmi: Redefine max message payload size

[cristian.marussi_arm.com]

Summary

The defined maximum payload size should not include header and status
field. Fix the definition accordingly, so that it matches the
definitions as can be found in the Linux SCMI stack.
The semantic of this define is important since it will then be possible
to override such values from devicetree, like in Linux.

Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Signed-off-by: Cristian Marussi <[e-mail]>

[andrew]

Does scmi_buf_get need to be updated with SCMI_MAX_MSG_REPLY_SIZE changing?

[cristian.marussi_arm.com]

In D47421#1081786, @andrew wrote:

Does scmi_buf_get need to be updated with SCMI_MAX_MSG_REPLY_SIZE changing?

No, it is just that these changes will affect the checks inside scmi_buf_get() to properly consider the status field and header.
BUT...I think I am still missing one more thing to consider, the overhead of the SHMEM layout for transport based on SHMEM...
(which I recently fixed on Linux...but I forgot here :<)

I will update this to address more completely this (to lowr basically the available PAYLD_SIZE properly when using SHMEM)
(and so I will update also the following patch that adds DT descriptors to change these props)

Thanks for the review !