fix(st-tamp): incorrect tamper event management

Fix some bad register usage and incorrect mask used
to manage the tamper events.

Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Change-Id: If38436150b80c5d5be95e2d802d9541bd6f5b2e4
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/251180


Tested-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>
Reviewed-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>