PS: ITS: Remove PS dependency on ITS

Currently, PS uses ITS services to interact with file system and
storage driver code. Update the implementation so that PS can use them
even when ITS service is not enabled.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I3edd88d77b01adbbce5729d41b8922d73cbd2ba1
6 files changed