[TF-M] Upgrade TF-M to C11 standard

Hello,

I'd like to check with the community about upgrading the compilation standard from C99 to C11. The initial trial looks promising, as you can see here: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/46212

Please let me know if your platform or project depends on C99 or if you would prefer to keep using it, and feel free to object to this upgrade. With no objections by Dec 21, I will merge the patch.

Thanks, Anton