[TF-M] [RFC] Build: Move UART symbols to SPRTL

Hi Platform owners, explicitly ST, Nordic and NXP owners.

Could you confirm that this patchhttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/6678 wouldn't break your platforms. The patch changes the build system to put the UART drivers to the SPRTL. It is important to improve the isolation implementations. For more details, please see the patch.

We will wait for one more week and merge it as is if no feedbacks. Any feedbacks from others are welcome always.

Best Regards, Kevin