SST: Rename SST(Secure STorage) to PS(Protected Storage)

This patches renames SST(Secure STorage) to
PS(Protected Storage) for files, folders and any contents
in files to align with the PSA Storage API spec.

Change-Id: Icf991f59512875780c159f22737b521b3b2d4924
Signed-off-by: Kevin Peng <kevin.peng@arm.com>