plat/st: protect uart during platform init

Protect the UART instance used for serial boot
with UART used for console.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: Ieee1557b34e7baa81594c3fbf0513191737027bf
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180872


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