stm32mp1: allow to override MTD base offset

Add an external parameter STM32MP_FORCE_MTD_START_OFFSET that
allows to override the default FIP offset or base offset
used to read the first programmed image. It can be used for NOR,
RAW_NAND or SPI_NAND boot device.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: Ibe664aae0e5ee90dd6629e544c9e034d751fffed
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/193647


Tested-by: Lionel DEBIEVE <lionel.debieve@foss.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@foss.st.com>
Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>