stm32mp1: invalidate download buffer cache

In case of programmer load, we must invalidate cache
used for the download buffer.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I372e3583e8964c70a71e2677a26dca74eb307bc2
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/183629


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>