CC3XX: Minor build/doc fixes

Fix some minor issues in the cc312-rom driver
such as:
- Make functions which don't return values void
- Fix a couple of documentation issues
- Fix functions which may return unitialized values

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>
Change-Id: Ica2bd320b5b72d3663264ce04fb3a2b1f009b449