[TF-M] Renaming SST to PS

Hi,

Please be aware that there is a ongoing changehttps://review.trustedfirmware.org/c/trusted-firmware-m/+/3597 to rename the SST (Secure STorage) service to PS (Protected Storage). This is to align with the PSA Storage API spec. The SST service is the implementation of the PSA Protected Storage service. The change renames lots of files, API names, build configurations etc... Lots of impacts are expected. Please do get prepared. Another notice after merging the change will be sent too. And, comments are always welcomed.

Best Regards, Kevin