[TF-M] PSA tests: NVMEM section

According to psa-arch-tests[1] the tests need an NVMEM area to write its states to. "Non-volatile memory support to preserve test status over watchdog timer reset. Each byte of this region must be initialised to FF at power on reset." I could not find any code that does this in the TF-M or PSA repos. How do other platforms reserve and manage this 1k area.

[1]: https://github.com/ARM-software/psa-arch-tests/blob/master/api-tests/docs/po...