stm32mp1: save boot interface in a static variable

In stm32_get_boot_interface(), there is no need to read the backup register
in which the boot interface and instance is saved. It can be saved in a
static variable. It will save time if it is used several times.

Change-Id: Ida5cbfeb9449d92bdb4f725eb6f6f1c4704282fe
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188319


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>