Add function stm32_get_boot_interface to get the current boot interface
from information saved in the TAMP register.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I6e99fe172d895fd9d8dcb8d827b949d0e89b7879
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180870


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>