[TF-M] Deprecate AN539 platform

Hi Everyone, As mentioned in https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/tree/docs/contri..., we would like to propose the deprecation of AN539 MPS2 platform and remove the same from TF-M master after next release. As per the process, this proposal will be open for discussion for a period of 4 weeks and if there are no major objections, the platform will be marked as deprecated.

Thanks & Regards Soby Mathew