[TF-M] Remove Customized Region Name Prefix in Linker Scripts

Hi,

The existing linker definitions abuse prefixes such as ER_ and TFM_. Some names do not have a clear scope, which reduces the readability.

Here are some patches to address the problem:

* Patch 10383https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10383: TFM_UNPRIV_CODE --> UNPRIV_CODE * Patch 10384https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10384: TFM_SP_LOAD_LIST --> SP_LOAD_LIST * Patch 10385https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10385: TFM_SECURE_* --> SECURE_*

Please help to put comments and inform us if your platforms couple tightly with the symbols defined in LD.

Thanks, Xinyu