feat(stm32mp2): add minimal support for BL31

Add the required files to compile BL31 on STM32MP2.
Update BL2 configuration to load BL31. The platform boots until BL31,
but stops here as no other binaries are loaded as DDR is not
initialized.
At runtime, BL31 will use only the first half of the SYSRAM, the upper
half will be used for non-secure DMA LLIs. To be sure nothing from this
area is still in the cache, invalidate the upper SYSRAM before enabling
BL31 cache. BL31 should then map only first half of the SYSRAM. But it
must temporarily map the upper half read-only, as this is where we will
retrieve BL2 parameters, used to fill registers for next boot stages.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>
Change-Id: Ie91527a7a26625624b4b3c65fb6a0ca9dd355dbd