plat/st: add boot from UART support

Handle boot from UART with STM32CubeProgammer with mmap io.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: Ibd719dd46a11da78633728675ef6639635b6cf67
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180874


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>