plat/st: invalidate data only when binary is not skipped

Fix a Data abort while trying to invalidate data at address 0.
When the partition is skipped, the invalidate must not be done
as the bl_info are not filled.

Change-Id: Ib90f1a7ac55648a9228e6dc53b875265a4eede32
Signed-off-by: Lionel Debieve <lionel.debieve@foss.st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/214784


Tested-by: Nicolas LE BAYON <nicolas.le.bayon@st.com>
Reviewed-by: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Nicolas LE BAYON <nicolas.le.bayon@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/224185


Tested-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>
Reviewed-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>