[TF-M] Adjustments for SVC number assignments that impact platforms with platform-specific SVC

Hi,

We're making some adjustments for the SVC number assignments. I'd like to collect some feedbacks, especially from downstream platforms that have your own SVC handler because corresponding changes of your SVC numbers are required when these changes were accepted.

Originally, The SVC numbers are categorized by their values, for example, numbers less than 0x40 are PSA API requests and numbers larger than 0x80 are for handler mode. This is not convenient for adding new SVC because it's not intuitive to find a proper value range. More importantly, it's easy to create clashes between TF-M and platform SVC by mistake as they are defined in different files.

So, we propose to divide the SVC number bits into different fields for distinguishing different types of SVC. Please see herehttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/20644/1/secure_fw/spm/include/interface/svc_num.h#12 for the detailed descriptions. With these changes, it's much easier to put the SVC numbers in the right range and avoid duplications after setting the bit fields to right values.

Thanks, Kevin