stm32mp1: move boot interface get and save functions to private file

Those functions are not directly linked to context, used for low-power
states. They are moved to stm32mp1_private.c file.

Change-Id: Id4b164aa078077fcf90557bedc2f14c4969fe5da
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188654


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>
Tested-by: Lionel DEBIEVE <lionel.debieve@st.com>