[TF-M] Arm platforms moved into arm folder

Hi,

We have moved all arm platforms into arm folder which will make the platform folder more clear. One thing needs to pay attention to is that you need to change the TFM_PLATFORM when building arm platforms.

For example: '-DTFM_PLATFORM=mps2/an521' --> '-DTFM_PLATFORM=arm/mps2/an521'.

It does not influence partner platforms build commands, only for arm platforms.

Best Regards, Summer