[TF-M] [RFC] Remove unused section 'TFM_UNPRIV_DATA'

Hi,

The existing section 'TFM_UNPRIV_DATA' now holds nothing - it was designed for holding some data belong to the unprivileged code block, while now the unprivileged code is required as read-only and no place for its read-write data.

Remove this section as it is unused; also remove those code objects exists in the unprivileged code section but has read-write data: 'platform_retarget_dev.o' and 'device_definition.o'.

This patch changes some sources inside the platforms, so the platform owner please help to check if that matters:

https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/7754

Thanks.

/Ken