[TF-M] TF-M Profile Small supports SFN model

Hi all,

I'd like to ask for your help to review the patch which enables SFN model in TF-M Profile Small. SFN model is defined in FF-M v1.1 spec. With this patch, Profile Small still enables Library model by default and users can select SFN model instead via build command.

Patch: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/14500 TF-M Profile Small document update: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/14501

Any comment or suggestion is welcome.

Best regards, Hu Ziji