feat(stm32mp2): add BL31 device tree support

BL31 will need to access a device tree for several configurations (UART,
GIC, OTP mapping...).
Create a BL31 device tree (SOC_FW_CONFIG). It is loaded in DDR, in a
spare area.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>
Change-Id: I320a05859e1aa3dd8db9a274e7201075a8c250c2
11 files changed