feat(stm32mp1): adaptations for STM32MP13 image header

The header must now include by default at least an extra padding
header, increasing the size of the header to 512 bytes (0x200).
This header will be placed at the end of SRAM3 by BootROM, letting
the whole SYSRAM to TF-A.
The boot context is now placed in SRAM2, hence this memory has to be
mapped in BL2 MMU. This mapping is done for all SRAMs in a 2MB area.

Change-Id: I50fcd43ecd0ba2076292b057566efe6809b9971a
Signed-off-by: Yann Gautier <yann.gautier@st.com>