feat(stm32mp1): enable BL2_IN_XIP_MEM to remove relocation sections

Because the BL2 is not relocated, the usage of BL2_IN_XIP_MEM
can be used. It reduces the binary size by removing all relocation
sections. XIP will not be used when STM32MP_USE_STM32IMAGE is
defined. Introduce new definitions for SEPARATE_CODE_AND_RODATA.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: Ifd76f14e5bc98990bf84e0bfd4ee0b4e49a9a293