build: remove experimental mark for PSA FWU support

The PSA FWU support is significantly matured in terms of
its design, threat model documentation, and implementation
as well as its adherence to the latest PSA FWU specifications.
Consequently, the experimental mark for this feature has been
removed.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: Ia6611e0c71485413b0da885f41ac59c93038e480